RE: Futaba receivers
Probably not a big deal, I see the rx is rated up to 6 Volts. Should be fine with no more then 6 Volts, your standart servos are probably ok with that.
I'd make sure it really does not put out more then these 6 Volts, it could cause damage to the rx or servos.
I have fried a servo this way, it shortened out and took an rx with it...good thing it happened on the ground.
So it's up to you, keep in mind that most regulators will put out a little higher voltage then they are rated for.
I think a Futaba system is totally fine with a 5 Volt regulator, safer then overloading the circuit.
You will get many opinions here, so this is mine and I fly helicopters and up to 1/3 scale planes this way with no problems at all.
Others will tell you they hook up their 6.6 Volt Lipos directly with no problems. I wouldn't