Sean...It's a while ago, but I remember playing around with different taps, and not hearing much difference. My guess is that the proper tap will allow the amp to deliver its rated power, but as long as the actual power level is well below the rating, which is the typical case, things will be OK. In fact, using a 4 ohm tap for an 8 ohm speaker would provide a better damping ratio, which might be of greater usefullness than unused power capability.
One good thing about an output transformer is that you can accomplish a lot of different things depending on how you use the various windings. In one case I rewired an amp with the 4 ohm tap grounded, and the speaker connected between what was originally the ground and the 8 ohm tap. This gave me a 12 ohm tap, which is what I wanted (Can't remember why). I also rewired so as to invert one channel of a stereo amp, so that I could drive a center channel speaker bridged across the stereo amps. Most interesting was a way to modify a monaural amp so as to be stereo, without adding any more tubes! The output stage was modified so that the complementary tubes were driven in both common and differential modes. A second audio transformer was driven from the primary center tap of the original transformer. One transformer output was Left + Right (the original mono) and the other was Left - Right. I forget now how these outputs were matrixed when the speakers were hooked up, but the end result was stereo, from a mono set of tubes. Clever.