Ann Marie from futaba once told me that all there receivers are rated up to 7.4 volts. A fully charged 6volt pack will do just fine.

Like Wgeffon mentioned a regulator will give you constant servo characteristics throughout the day but this will only be noticeable on more advanced type aircraft flown by more advanced type fliers..

I run 2 6volt NIMH 1650 packs plugged right into the receiver with hitec digitals and the run great.