Without any cooling you probably will have a hard time getting your Voodoo2 to run above 100 MHz. I had some luck running uncooled at 100 MHz, but freezes were still common, and games like Unreal really gave me a bad time. At 105 MHz uncooled I could barely even run Quake 2. So if you don't plan on cooling your Voodoo2 at all, I would say you should shoot for about 95 MHz for stability, or possibly up to 100 MHz if your PC is already well cooled. If you really want to get some results, COOL your Voodoo2.
Stability is the obvious reason.
Running at the default 90 MHz the Voodoo2 already produces incredible amounts of heat. With good cooling the Voodoo2 may remain stable at speeds of up to 109 MHz. If you have really good cooling 109 MHz will be attainable. There are several methods to cooling the Voodoo2. To eliminate the heat you must first know what causes it.