Joe:
You can run most standard digital servos up to around 6.5V without a problem. In particular, JR digitals hold up just fine on regulated supplies in that range. You could set up a mixed standard / HVsystem that picks that as a compromise setting. The nice thing about regulating LiPo packs, even for an HVservo, is that you get the same performance in terms of speed and torque output throughout the usable discharge curve of the battery pack. It's really no different than what we did with 5 cell NiCad or NiMh packs a few years back, except that now the performance curve has shifted higher. Even if you go all HV, getting a regulated setup that is set somewhere around 7.5V, or slightly higher will give you that kind of consistent, high performance. There's something to be said for simplicity, but there is even more to be said for a setup that protects you against switch and battery failures.