Yes, I do it quite a bit. Maybe not all the time, but more than I should. Nearly always, it's not because I didn't understand the joke, but because I only heard either the first or second half of it, or some other small part of it, but didn't hear the rest or wasn't paying attention then. I usually laugh anyway, despite that, because usually other people are too (if any are present), it gives the illusion that I was paying attention the whole time, I find a lot of stuff funny, even bad jokes, and probably would have still found it funny if I was paying more attention, and it shows that I appreciate their joke.