Pulling bullets won't, by itself, make them smaller, but the seating in the first (and second) passes can. In general, you're sizing in a few thousandths of neck tension, as they say, "something's gotta give..."
I've never found myself in a position where I had a LOT of rounds to pull, so I'd have a LOT of bullets to re-use. So for just a few bullets, I don't usually bother reusing them. Won't hurt anything to reuse them, I just typically don't, because it's not usually worth the trouble for 10-15 bullets. For more than 100rnds, where we're talking about $30-50 in bullets, or more, I'd be motivated to reuse them, but then again, you're talking about a lot more time to pull the bullets - and I've just never found myself in a position where I have had 100rnds or more of improperly loaded/sized ammo. So usually, I just discard of the bullets and figure I've wasted more money on more foolish things.