I generally inflate tires to the maximun pressure listed on the sidewall but as mentioned, only if the tire is in good condition.
Interesting side note:
I went hiking one day and when I got back to the car at dusk, I discovered I had a flat tire. I pulled the space saver outa the engine bay and it was flat too. I was 20 miles back in the woods and didn't have a coat, no food, nothing. I didn't want to ruin my good flat tire so I bolted on the spare and drove on it determined to get closer to the highway. Anyway, I ended up driving all the way out on it and then another 20 miles, ON THE FREEWAY! at 50MPH. I got home and the sidewalls were damn near worn through but it didn't even break the bead! Them suckas are tough!
BTW-I reinflated it (not to 60PSI!) and it would hold air with a slow leak!