The exception is that there are already 2 limpers in the pot, in which case it's fine to limp behind some straight-draws and flush-draws, but gut-shots should typically be folded. It may seem attractive to be dealt 4-to-a-straight or 4-to-a-flush, but we usually won't hit them often enough to make up for our preflop investment. In most cases we should avoid drawing to flushes and straights.