Last time I looked in the back of my Saturn TX there were 8 cells. Still, time moves on. Old RC gear was designed around running on 6 volts, because it was so convenient and be arrived at with many differing combinations. Modern gear with logic chips often (but not always) has a 5-and-a-bit volt limit, which it is safest to assume is true. I would either feed the radio from 4 rechargeables, the BEC in the ESC, or a separate BEC, as appropriate.
I've never been a fan of using dry cells for the RX, since that supplies the servos as well, and when they want current, they tend to get to the limit of what dry cells can manage. This results in the voltage dropping under load, and, since the electronics all relies on having a stable voltage, results in strange jitterings happening.