What I was taught (a long time ago) with regard to fuel was
1 - You must depart with a plan and fuel sufficient for the plan plus 30 (45 if night) minutes of fuel plus what ever additional reserve you as the pilot feel is required.
2 - If at some point in the flight it becomes clear you will arrive with materially less than the reserve amount the pilot should divert.
3 - If nearing the end of the flight (ie. diversion will not save much fuel) and the pilot expects to arrive on his reserve this would be expected to result in a 'Minimum Fuel' advisory call. The meaning of this is, I do not require priority handling for safety, but if I get any material delay I will need to declare a low fuel emergency (PAN PAN or MAYDAY in Europe - depending on the precise nature of the situation).
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To the question, voluntarily poncing about in the circuit while burning into your 30 minutes (eg. lets do a few practice approaches before we land) has every potential of a Careless and Reckless charge if the FAA notices. However, there is no hard and fast legislation (like much of the FAA system), so they would view this activity in a typical spam can (ie no real certainty as to the amount of fuel) much less favourably than a highly instrumented aircraft with capacitive fuel gauges/fuel flow, etc.(i.e. high certainty as to remaining powered time).
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Reminds me of a line from one of the Vietnam war pilots, 'I still had 4 minutes of fuel - not yet at low fuel emergency, so I let the B-52 do the 'dreaded 7 engine emergency landing' ahead of me'!