I wanna say 6mm per side is the generally "safe" rule, that's about the max we would do at the shop I was working in. Working on friend's bikes, I've definitely done some shoe-horning of tires into frames, but I've always been very upfront about the drawbacks and usually recommend going to the next size down when looking for new tires. I've become a little more tempered in my shoe-horning if for no other reason than it's easier to get the wheel in and out with more clearance.