I've ben running robot servos in my RC's for years, Have one in my 17lb Losi XXL with no servo saver and Castle/BEC nearly 4yrs and still going strong.
A fully charged 2s lipo is at 8.40volts but only for very shortly befor voltage diminishes to a more more steady 7.4 or so. If the RX your useing can handle 9.0 volts (Spektrum) then you have no worrys.