Coils generally don't go bad.

I'd say no - if a coil is running fine, I wouldn't consider replacing it. Maybe if you were trying to return a machine to 'as new from the factory' state, you could, but I think that would be more 'to say I did', rather than for much practical benefit. Besides which, it's generally pretty easy to replace coils with the playfield in the machine.

On the other hand, you could certainly look at replacing linkages (e.g. flippers), coil sleeves, bent leaf switches, and maybe even limited-life electronic components like PSU capacitors, DMD etc if you were really keen. I'll take your old DMD panel off your hands!

As long as the coils aren't swollen, don't restrict plunger movement, aren't melted/burnt, and give the proper resistance reading, they should be fine.

I've seen some folks replace all the wrappers in high-end restores to make them look "new".

http://www.pinballmedic.net/coil_chart.html

One sign of a bad coil is not being able to easily remove the sleeve. That usually means it got overheated at least once. This is another good reason to replace all sleeves when you are doing a restoration.

Like John_I said, if you have a tough time pulling the old sleeve out (or worse, just sliding the new one in) then the coil got too hot at some point.

But generally - Sleeves and stops. Are the big things.

Extra - Check the linkage to make sure the holes are round and not oval. Check the plunger for mushrooming. Check the bushing for cracks.

If everything else looks, just clean.

When doing a restoration I clean my coils with an ultrasonic cleaner and replace the wrappers. If "warm and fuzzy" equates to clean and restored then I am guilty.