First of all, you should not have 'full flap' in a C172 at 500 feet. I know they teach students these days to configure once they are established on final, however this will involve you 'dragging' it in all the way down final. This is 'poor' airmanship when considering the following aircraft, and a recipe for disaster if the situation you questioned happened for real. It may be okay at a GAAP (class D) aerodrome where everyone is doing the same thing, if you fly in the real world and do this with an RPT aeroplane up your behind, you could become an accident waiting to happen - especially if you were not making the required radio calls (which has happened to me on occasion).
Energy management is a crucial aspect of airmanship which is not taught. The trick is to fly the aeroplane all the way to the ground taking into account the current conditions without making large power changes, adding flap as required.
If you are following a slow aeroplane on downwind, don't fly wide, stay in gliding distance of the field, slow down and extend 10 degrees of flap. If you have the circuit to yourself, stay clean (not 3nm which seems normal for lighties these days) and extend flap as you require.
As a rule of thumb when I was flying piston VFR aeroplanes, I would not configure until 300 AGL (gliding distance of the threshold). Stabilized approach criteria is something you should be worrying about when you are flying an aeroplane where this is important.
In regards to your original question, you do as a Pilot what is required to fly your aeroplane safely in regards to the current situation. If this calls for raising flap to extend your glide to make the field - then do it. If this requires side slipping a C172 (which is prohibited in the POH with full flaps) then do it!