Flying with the master switched off simply powers down the gyros, amongst other things, and would lead an inexperienced PPL into a spiral dive and an instructor with no instruments to recover
The only flight instrument which is affected by switching off the electric power (in most light singles) is the turn co-ordinator.
I'm not sure about the wisdom of switching everything off - but my concerns would be centred more on how (in)visible I become to other aircraft (a small concern, since I am looking out, it's a big sky, and it's only for a few moments) than the loss of a turn co-ordinator.
FFF
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