I have a road bike (two, actually, because it rains a lot here and one of them doesn't like that) so the psi numbers won't carry over for you, but I think the point is valid. I run 110 psi in the front tire, and 120 in the rear. If either one of them is off by more than 5 psi, it will bother me.

If the back tire is soft, it feels a little, I don't know, weird in turns. It's like wide turns happen just a hair more slowly, and, also, it can feel like it wants to fishtail. The front one feels weird in its own way if the pressure is wrong, but I can't really figure out how exactly to describe it. This is especially unnerving because I got used to the way the bike handles, and I've come to expect that, so when it doesn't behave the way I'm used to, I'll worry just a little bit about whether there's a mechanical problem.

The flip side to being so neurotic is that if this all doesn't bother me, then I know everything is just right. And the reason I'm boring you with this is that you should see if you can judge tire inflation by feel instead of by look. (A gauge is best of all, of course.)