Ive heard from many instructors here at my part 141 school that what has been said so far is consistent. MDAs are set in stone. Under no circumstances should the aircraft be below that altitude unless going for a landing. This is generally because you are leveled off at that altitude as you make you way to the MAP so there is no inertia to carry you down.
However, for ILS you make you decision at 200 feet. Now with small GA aircraft, generally you will pop right up and even 20 feet is not much of a loss. Now one instructor pointed out much larger aircraft. Suppose a 747 was doing an ILS approach at an airport that only had Cat 1 ILS. He shoots the approach down to 200 feet, no field in sight and he puts the thrust levels at full... there is going to be some spin up time plus general physics of stopping that honking thing from going lower... it could go quite a distance below the DA!