I agree. To me, denatured alcohol is another work bench 'essential' liquid. Denatured alcohol shouldn't damage good condition
varnish or paint finishes. Mineral spirits are too weak to take the old varnish goo off spools without way too much work. Denatured alcohol is a 'stronger' solvent. By starting with water and a good detergent, like Dawn dish soap, then mineral spirits, and finally denatured alcohol, you can clean almost anything.
For what it is worth, I've had very good luck removing old labels and tape residue with mineral spirits.
For System reels I've had success cleaning the working parts with Ron's vinegar treatment. The reel on the right hand side of the photo looks beat up (it is). I thought it was only a parts reel, but I put it through the full Bulldog™ Procedure
anyway. I wanted clean parts after all. The reel came all
the way back. With the crud gone it is as comfortably smooth as an old pair of blue jeans.