Very easy to back fill through the hoses, with no worry about if the impeller will suck up AF or not, or if the thermo is open or not. You don't actually have to remove the thermostat when you backfill, I disconnect the big hose from the thermo housing to the water pump on the front of the engine, at the thermo housing, fill thru the hose (still connected at the water pump end) till some AF comes out of each block drain, replace drains, and then fill till it appears at the thermo housing, takes about 2-3 gallons of AF that way. I fill the manifolds till it just starts to spill out the exhaust. Same thing with the raw water intake hose. Make sure you clean up any AF on the ground even if it's no tox. I use the -100 so I don't have to worry about the possibility of trapped water, or if we get an unusually cold winter. I've done it this way for the past 5 seasons and the thermo housing I replaced in 2003 has very little rust for a salt water motor, so I do think it helps.
PS the first year I tried the suck up the AF, hope the thermo is open method, checked the block drains and found not much AF in there, so drained the block and backfilled, that's why I've always done it that way since then.