Which manual? But as long as your receiver and servos are HV then you are OK. Now there is 4.8V, 6V and HV which is I think around 8.4V. But when soneone refers to HV, they are really reffering to 6V. Most servos are rated at 4.8V / 6V for tourqe and speed. Always, higher voltage will give you more tourqe and speen on your servos.
There are regulators out there and I have a couple, but I dont use them. I just run the 6V rec, servos, and batt. The only regulator I use is the IBEC that acts like a regulator for the ignition module and kill SW, with a LED for on indcator. The Life battery is the most stable voltage source out there. In fact I think it is somewhat of a downfall. Because when it first starts out at about 6.8 or so it will drop to 6.6 fairly soon and stay there until about ready to die! and then it will fall off very quickly. some meters will tell you how much is left in %. But a standard volt meter will not tell you really where you stand. You have to charge the battery back up to see how much you used to really get a picture. But, I have used Life batteries in most of my gassers and after 4 or 5 flights I normally only use about 500ma for my day of flying. Not really even close to having to recharge.
Anyway, I picked up the IBEC at GravesRC today. Last one they had. There are a lot of choices out there and I tend to stay with what I have used and proven to work. Been flying my giant Revolver for two years now without a failure. And it looks great still. Had to replace the tires, worn out tread. It's my go to flyer with hundreds of flights and it is set up just like im setting up this Citabria.