Lancelot and others: Unless the springs are rusted up real bad, I don't think you need to replace them. The biggest cause of buttons sticking isn't the strength of the spring, it's gunk built up between the two plastic parts of the button (ie: between the part that moves down, and the part that's around the part that moves down).
I may be wrong, as I haven't been through 60 to 80 buttons as Lancelot has, but I've had a few used ones stick, and what I did was pop the button part out of the holder part, used some warm soapy water and cleaned the suckers. Now they bounce back just fine!
~Ray B.