John,
You only have so many mixs. using one for your dual elevator servos may shorten your options later. and if you really are going to want elevators to act as ailerons for roll rate, then you want an 8 channel receiver so you can use ailevator.

the likelihood of needing every mix, though, is pretty slim. if you don't really want hte aileron-to-elevator mix (that model has no need for it, IMHO) then i wouldnt worry about it.