DSM2/X - when are channels chosen?

Passing thought; at what time are the channels (bands? frequencies?) chosen by the TX? ie., DSM2 chooses 2 "free" channels, DSMX supposedly 23? But does this happen every time you turn on the TX, or does it only occur the first time you bind the RX (for fixed ID)?
If the latter, just thinking that if you bind at home the first time, then take it to a field that's noisy, then that could wipe out your signal...

Every time you turn on the transmitter.

Edit:

Thanks Bruce.

Thanks aMax, ironically it was a post I came across from rcmodelreviews that triggered the thought, where he found that sometimes DSM2 would give a very narrow separation between selected channels, then he managed to completely drown out both signals with a video feed.
The more I thought about it the more I figured they (Spektrum) couldn't have been that dumb to only ever select the channels the very first time it bound...