How can I fix a bathtub that was not set properly?
If you look at the underside of the tub (through your proposed hole) you should be able to tell if it was meant to sit on the subfloor or not.  Some kind of wooden frame will be partly embedded into the fiberglass to help support it.  If there are "skids" that should touch the subfloor but do not you can use shims to support them.

I had a shower enclosure that was not well supported underneath.  It eventually cracked and leaked on the ceiling below.  When I removed it, it was obvious that the enclosure was sitting on the floor in the front and hanging from the wall in the back.  The reinforcements on the bottom of the enclosure were not touching the floor.  I don't know if they were meant to or not.