I was once rapped over the knuckles by a very experienced Flight Test Examiner, one Eric Thurston, for lowering flap during a turn in a C172.
He said that you should always make changes to flap settings with wings level so that if one flap lowered and not the other, you'd be made instantly aware by the uncommanded roll and hopefully address the situation. If you're in a turn (especially base/final as we were) you'd never be able to sort it out. That's also a good reason for lowering and raising flap in stages, rather than all at once. Of course, NEVER just flick the switch and rely on the next detent to hold it, positively check the switch position.
That was 15 years ago and I've always followed his advice since.
Although the PA28 system seems intrinsically safer, I still follow the 'wings level' routine for any flap change with this type, too.
Although I only flew with Eric on that one test (he passed me, for some strange reason!) I do recall a number of aspects of the flight that have stayed with my flying. He struck me as a remarkable bloke. I don't know if he's still around, but aviation certainly benefited from his work!
We had an accident with our Rallye when someone allowed the switch to go all the way through, but that's another story!
Cheers,
The Odd One