I have 7.1.2 on my 7100/66. 7.6.1 with Speed Doubler is a much more usable OS. (The 68k emulator in the first generation of Power Macs was really bad, and Speed Doubler replaces it with a much better software emulator.)

In general, I use System 6 for my 68000 machines, 7.5.5 for my non-32-bit-clean machines, 7.6.1 for my 32-bit clean 68ks and NuBus PowerPCs; 8.6 for any PowerPC less than a G3, 9.2.2 for G3 or higher. OS X 10.2 on beige G3s, 10.4 on my other G3s and G4s. (Other than Leopard on my MacBook Pro, eMac, and PowerBook G4.)