I don't put a collar on mine, but I'm in the fortunate position of having a safe enclosed space for the cat to go out in, and he came from CPL with a microchip already 'installed'. Having said that, I've had to take him to the vet's with a cut in his armpit (!), so I know that even in a space where I think he's safe, he's liable to get into a scrape.
Plus, there's the fact that he's a semi-LH with a big fluffy ruff, so putting a collar on him would make him look like a sunflower.