I've done a lot of drop-D tuning, mostly for metal and rock riffs, but I've never tried drop-C. It seems like the string would fall right off the guitar, or at least you'd get nasty buzz. I'll have to give it a shot next time I restring.
Question, though: Do you think dropping to D/C and then tuning back up to E repeatedly will shorten the life of your strings, or even risk breaking them? Or is it no big deal? I like to occasionally drop to D when I'm playing for friends, just so I can lay out some Pantera or Tool, and occasionally I go 1/2 step down on all the strings for some GnR, but I haven't tried dropping down more than a full step on any given string. I restring regularly, so I'm really just curious if dropping hurts your strings at all.
quick edit: What about tuning up? How far can you go, and is there any point? I can't think of examples where you would need to tune up.