I've always wanted to ask but keep forgetting, do you replace all coils with new when rebuilding or just those that are obviously cooked? What do you look for and consider to decide its time to replace a coil? Difficult to remove sleeves, low resistance readings?  

The first thing I do is check that the correct coils are in the correct position. Sounds basic, but most games have the incorrect coils. Most commonly - Flippers. If the coil sleeve is burned or difficult to remove, I check the resistance of that coil, and if it is not good, I throw it out. I check the resistance of all coils. If the wrappers are falling off, or if they are generic labels, I print off new labels and install.