Mass has cops that look for that stuff, so I put them on the second we get them. We have two cars and we keep them current.

Last year I drove a car with expired stickers for 6 months. Didn't ever get ticketed for that. Even the time I got pulled over for speeding (which I also didn't get a ticket for) they didn't bust me on it. The car belonged to my mother and the plates were from a different state than I used it in, so that may have helped. Either way I gave it back eventually. On my own car I keep it up to date. :)