Friend away! I usually friend back, as it's fun to meet new people who share my interests. If we hail from the same fandoms or we've friends in common I'll probably add you without question. If we don't, speak up and tell me why you friended me! I'd love to know.
One exception: I post private scans and translations on my journal, for friends only. So if you're a member of half a dozen scan share comms, I'm afraid I can't in good conscience add you. I respect the artists, and frankly you should too. (It should go without saying, though sadly I've found it doesn't, that I won't add you just so you can see the scans. Don't ask. Nor do I keep anyone on my flist who hasn't friended me or who has defriended me for precisely the same reasons.)