Be sure to bypass your water softner if you have one, if you fill from an inside tap. Switch on the bypass.
Brewman
Seems like you would have to do this in advance, or the tank would be full of nice hot soft water.
I have always discouraged people from filling with hot water. The tanks are usually full of grit and calcium just at the bottom, and some people use the clean out as a hot water tap. That basically equals cleaning your water tank out into your spa.
Assuming you use a regular hot-water tap, such as the one near your washing machine, you avoid dumping the tank into your tub, but you still have to be sure not to put in water that is hot enough to damage the surface or trip a high-limit switch somewhere.
Finally - I find that the 40 gallons or so of hot water only seems to warm the tub a tiny bit. Unless you are willing to wait for the tank to reheat over and over, you will not likely save much time in getting into hot water.
If you have a big boiler with a heat exchanger, and you can watch the water temp carefully, and if you are sure you're not getting soft water, and if you start with cold water in the bottom of the tub to keep from steaming the bottom, then go for it.