What I think matters most, is not that a game is clearcoated, it's who did it and how well was it done. If it was done poorly, it devalues the game greatly, if it was done right (HEP) it increases the value. Most playfields that have had restoration work and touch-ups are cleared to protect the touch-ups, so they should be judged on the quality of the touch-ups and clearcoat and valued accordingly. As far as the shine, generally speaking, the newer games look great with a high sheen, the older E.M's probably look better with a semi-gloss or satin sheen. Like with everything, opinions vary, these are just mine

I also believe that's the important factor. If a machine has good clear mylar on the playfield just keep it clean. Speaking of the value, any machine that looks good enough to eat off of has more value.

In kawikids case it devalued the machine. Paragon about 2 weeks ago.

If you have to do restore work, the game is already non mint, and a clear coat would add value.

If the playfield is a reproduction, then a clear coat will eliminate the need for Mylar and add value.

If the playfield is totally mint, collectors quality, then you might actually lose value by clear coating it; as serious collectors want the factory finish.

Just like a classic car, the mint factory paint is worth much more than if the car has been re shot.

If it was cleared at the factory and you're tearing it down, a little extra clear (done properly) couldn't hurt.

Yes. I cleared my Tron LE a few weeks after getting it.

I had some fellow pinsiders do artwork and clearcoat on my RFM recently, It looks REALLY sweet, like a sheet of glass! Very happy I went through the process

Chris at HEP is your best bet.