I'm assuming we are talking about acoustics here.
In general, our advice at the shop is that if you are using lights, you can tune to standard pitch, but if you want to use mediums you should tune down 1/2 step. A lot of guys like to tune down because, at pitch, 12 strings can sound awfully jangly, which can be a bit much. Feel free to experiment, but unless you get a Bozo, don't tune it up to pitch with mediums. If you get a Bozo, you can use heavies at pitch - he likes to build them kind of like a heavy tank, and they can stand up to just about anything; you'll need a to use a sledgehammer as a pick to make them sound good, but once you do it will sound amazing! (I should make clear here, I like Bozos - I just can't play hard enough to make them sound good! Oh, and his tastes in inlay are, well, over the top.)
As for production 12-strings, look into old `70's Guilds, or the Taylor Leo Kottke model 12 (older ones are better than the new ones - if you can find one before they started using the NT neck joint, that's the one to get). That is, by the way, the only Taylor I actually like, but it is a very nice 12 string; just be aware that it is designed to be tuned down a lot (Leo is tuning down two steps to a C these days).
Light
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M.K. Gandhi