Gazzer

No!

A 4 cell pack at 4.8v is...just about flat!

Off charge around 6.1v is normal and after settling 5.7v.
Under load voltage does not want to drop much below 5.0v, so between 5 and 6 volts is fine for all JR digital servos, which as you rightly say fill 99% of my Jets, large scale and helicopters.

I use Graupner 4.8v (4 cell packs) 3300 or 3600 NiMH on some PB installations like my EuroSport twin and Lightning. Good quality 4 cell packs are fine-avoid budget cells...even brand named versions, if they are cheap then thats exactly what they are...
If you want top speed and performance through the PB units use 5 cell packs or PB's excellent Li-Po packs as this is the only way you will get the regulated 5.9v output-I use the 4 cells as none of the quality battery people I deal with offer 5 cell packs.
I'm ready to try the PB Li-Po packs in one of my next projects.

Regards

Dave Wilshere
Modellbau Deutsch, Graupner, JetCat, Airworld, BVM, Ultra Precision and other quality brands!