I usually use a schematic as a last resort. Most EMs I buy are tired and need a good going over. I like to clean and inspect everything and adjust where necessary. If you go through a game top to bottom you generally only need some fine tuning when done. You'd be surprised all the odd things you find when you do it this way. Because of this I can't imagine only fixing what is preventing a game from playing at the moment. So many potential lurking issues can be corrected when you are more thorough.

I always felt like I was pretty thorough but a guy like Dirtflipper takes it to the next level. He goes as far as disassembling all the relays puts the plates in an ultrasonic cleaner. They turn out awesome though. One day I'd like to get to that level.