hi wicker,
matter of personal choice really, but we always teach 10-12 degrees ( ours does'nt have the detent type lever ) , and as required for practice or necessity on finals.
in fact unless we are practising full flap approaches we generally use the take off setting ( we use 10-12 degrees for take off )for all touch and go's...reduces the workload.
i would prefer to leave it until then, so as you say, you get a better view of the runway and appreciation of glide profile on finals rather than on base, and this does'nt just apply to 172s.
its just like...some like a power reduction of downwind before the turn..thats not what we teach...for the reason you state ( possibility of stall/spin ), but also its when you turn onto base that you get a better appreciation of the runway and you aiming point..you might have a headwind on base or you might have a tailwind so the power reduction might have to be done right away after the turn or left a bit longer due to a headwind on the day....
flaps likewise are a matter of choice but i would certainly not be looking for anything more than 20 degrees on base.
gear up.....
the dean.