As far as I am concerned it is standard practice to switch to standby before changing sqwarks. I have never flown with anybody who doesn't do so and it makes good common sense all things considered.
The only draw back is you have to remember to switch back to ON. Something I forgot during by BCPL GFT, luckily the examiner either didn't notice or at least chose not to notice. It's not something you do twice after that.
I also thought that a 75 76 or 7700 sqwark remained on the radar screen even if you change sqwarks afterwards. Although ATC Babes post sugest otherwise. Can anyone clarify?
Also presumably there must be a small delay otherwise the radar would show constantly changing numbers as an aircraft (that hasn't changed to standby) changes sqwarks?