Actual voltage readings using a 4 cell 600 mAh NiCd pack to operate a GP Tracer (.40 size aerobatic airplane) with 5 servos measured with Hobbico digital voltmeter loaded for a 4.8V Rx pack. Each flight lasted around 12 minutes:

Initial voltage before flying: 5.3 V
After 1st flight: 5.1 V
After 2nd flight: 5.0 V
After 3rd flight: 4.9 V
After 4th flight: 4.8 V

The trend is obvious and typical of 4.8 V Rx packs.

There is a representative Rx battery voltage graph by Bob Kopski on page 102 of the latest Model Aviation that also clearly demonstrates the voltage trend (September 2004 issue - in spite of the fact that it is only July. RC mags seem to be in a timewarp). Bob writes that he stops flying at 4.5 V, but that gets a little too close to the edge for me. I stop flying at 4.8 V.