If all your servos and the RX are rated for 6v, by all means use 6v.
BEC's generally work with slightly less heat dissipation at 6v, and may be able to provide
just a bit more current.

Naturally, you can have servos that were over rated by the mfr for marketing reasons,
and may eventually have problems at 6v. If this is a possibility, 5.5v might be safer.