With a spread spectrum radio like the DX7, the reason you turn the transmitter on first is so that it can select a couple of open frequencies. After that you turn on the receiver, and it syncs up with the transmitter.

With that type of radio, there's no chance of it picking up stray signals if you happen to turn the receiver on first. What will happen is that the receiver goes into its failsafe mode, which for the low-end receivers is neutral surfaces and throttle to idle.