I like the idea of damage and weather, but I'm not a fan of the ultra carved-up armour that has become popular recently. I prefer the look of use, not abuse. I think you need to combine damage with wear - if they are all polished and newly painted then why didn't they repair thier armour? I just read the Forgeworld painting book and really like the verite way they approached weather (since they are historical nerds). I've gone for that look, worn paint, a bit of ingrained muck from hard soldiering. That way they fit in with the bases and poses, which I've aimed to be natural rather than heroic. It does mean that your minis will not look as white dwarf as others, and you have to be careful not to go overboard! But I think with the right combination of colours you can pull it off. It also suits my painting style which is fairly gritty. [URL="http://www.lounge.belloflostsouls.net/showthread.php?t=3163"]http://www.lounge.belloflostsouls.net/showthread.php?t=3163[/URL]
But that's only my opinion. What I think you need to do is decide how you want to your guys to look thematically: herioc, grunts, cartoon, hyper-real or verite.
The back story is that they worship their armour, so going into battle it will be clean and shiny (especially Black Templars!). But once meat meets metal things will get confused, and on any long campaign things will start to look shabby (I used to fix up old scooters and even well kept machines would start scuffing and and have mud caked in odd places - better still look at cars in carparks, and metal roadworks barriers and see how much damage they get from day to day use).
For some good work (thank you cool mini) on verite [URL="http://coolminiornot.com/218197"]http://coolminiornot.com/218197[/URL] is an excelent (upscaled) example. [URL="http://coolminiornot.com/index/whatc/SF/id/218153"]http://coolminiornot.com/index/whatc/SF/id/218153[/URL] is another. These are realtively tasteful, clean but not new, work but not totally cut up. This style suits both heroic and grunts, with more or less weathering. These guys have damage (rather than wear) which I didn't put in to my work.
For a more heroic, hyper-real aspect, [URL="http://coolminiornot.com/index/whatc/SF/id/59386"]http://coolminiornot.com/index/whatc/SF/id/59386[/URL] [URL="http://coolminiornot.com/index/whatc/SF/id/87600"]http://coolminiornot.com/index/whatc/SF/id/87600[/URL] [URL="http://coolminiornot.com/index/whatc/SF/id/228474"]http://coolminiornot.com/index/whatc/SF/id/228474[/URL] and [URL="http://coolminiornot.com/index/whatc/SF/id/68623"]http://coolminiornot.com/index/whatc/SF/id/68623[/URL] all use heavy damage over a (mostly) clean surface. This style has less wear, more damage and cleaner lines and a very modern comic feel. Before my comments I'll say I'm personally not a great fan of this style, so take what I say with a grain of salt . I find the disconnection between the damage and the cleanliness (and often the base) jaring. Also, the glowing eyes, except when done very (and I mean very) well, tend to detract from this more than in a true comic style (but can be done). It has a superficial air that does not sit right since there are gouges but no scuffs. Here there is also the issue of where damage occurs. Frequently it will occur in random places, but a lot in the above examples doesn't make sense (from a realise perspective). I tried to make my scuffing occur only where it would make sense (and I hope it largley worked) - there are a lot of pics of Lands Speeders with random damage to the trailing edges of their dorsal fins and I can't quite work out what caused it. And as someone below said, doing too much weathering looks cluttered and by making it hyperreal accentuates this. Of course sometimes I'm proven totally wrong and there examples in this style with all the things I shouldn't like where I like the whole model.
For a more heroic, cartoon style, [URL="http://coolminiornot.com/index/whatc/SF/id/192241"]http://coolminiornot.com/index/whatc/SF/id/192241[/URL] [URL="http://coolminiornot.com/index/whatc/SF/id/167218"]http://coolminiornot.com/index/whatc/SF/id/167218[/URL] [URL="http://coolminiornot.com/index/whatc/SF/id/158395"]http://coolminiornot.com/index/whatc/SF/id/158395[/URL] and [URL="http://coolminiornot.com/index/whatc/SF/id/210313"]http://coolminiornot.com/index/whatc/SF/id/210313[/URL] (and the White Dwarf style). These all tend to have little or not damage or weathering. I think this works better for the heroic type model as it does not even attempt to pay even lip service to the real world except for colour and texture. It is much more internally consistent.
But what it boils down to is what image you have in your head, your skill, what you will be happy with and how much time you can spend on it. I put together my guys pretty quickly one I got used to painting again with some simple tricks (two nights to do all the weathering - and that included at lot of drying time). If you are interested in weathering techniques I do recommend the FW book, it's the best book I've read on the subject (and my dad is a railway buff so I've had quite a few pass though my hands).
Black templars are heroic and anal about their armour, so clean works. But they are also knights and get in the thick of it so dirty and damaged works too (whenever I think of knights I get the image of the end of Branauh's Henry V with all that mud a blood).
If you decide to weather you should also spend some time wokring out the colours for damage, since the Black Templars are black so any wear and weathering will be harder to do neatly and lightly (since lighter colours will stick out). [URL="http://coolminiornot.com/index/whatc/SF/id/120258"]http://coolminiornot.com/index/whatc/SF/id/120258[/URL] is a nice (big scale) example I found.
OK, that was longer than I was expecting, so I hope some of it helps. A lot of it just my opinion so needn't really cound for anything, but I hope it gives you a different perspective for making a choice.