It sounds like some are mixing IFR and VFR procedures. FAR 91.175 are IFR procedures and a circling approach is an IFR approach. Do not think that once visual you are then cleared to do what you would under VFR. You are required to maintain MDA until "...in a position from which a DESCENT to a landing...at a NORMAL RATE of descent using normal maneuvers..." In the abnormal situation that a circling MDA is above the TPA then you should not descend to TPA unless you cancel IFR and the tower clears you to continue VFR. They are only going to do that if they have you visually.
Someone mentioned Required Obstruction Clearances for various approaches.
ROC circling = 300'
ROC VOR/DME = 250'
ROC NDB = 300'
ROC ILS = variable