Guys, yes the battery voltage will vary but it wont make any detectable difference to the performance of the servos. Even the top pros often use a 2S LiPo to power their HV servos, and if they can't detect any loss of performance then it's probably good enough for us too! For sure I can say from my own experience that there is no detectable difference as the battery voltage runs down, absolutely zero, nada, nothing..
Using a BEC and a seperate battery just adds unnecessary cost, weight and complexity, and a potential failure point. Also if you do use a BEC it needs to be a good one capable of supplying the high current these servos can demand.
If you want to use a BEC get an HV BEC and power it off the flight pack (no need for a seperate battery). BEC stands for 'Battery Eliminator Circuit', the whole point of a BEC is to eliminate the seperate Rx battery. The Castle BEC Pro is an HV BEC and will do a decent job, I use Gryphon Quasar HV BECs on a couple of my helis, they are good too and dont require a programming dongle like the Castle does.