It takes 10 minutes to drop the pan on a Toyota automatic. Zip off all of the 10 mm bolts with an impact driver (be careful if it's a super rusty rig), pry the pan off, clean out the gunk on the inside with paper towels and brake cleaner, replace with a new filter, then re-install. Lightly zip the 10mm bolts back in with the impact, then hand tighten for a few rounds. 30 minute job total from start to finish when the rig is already up in the air and you have the skids off. Add in another 15 minutes for cleaning FIPG off on a 4-speed rig. I did this job on a '02 Tundra with the 4-speed and we used a filter kit with a rubber gasket (same as the later 5-speed rigs) and it's been sealing totally fine. I've done the job at least 4 times on my A750F GX for various reasons (2 fluid exchanges, Nomad valve body install, and trans harness replacement).
If you're already getting messy and doing a full fluid replacement from the front of the vehicle, I don't see why wouldn't wouldn't drop the pan to get the gunk out. Otherwise the gunk is in there and will contaminate your new fluid. You'll be surprised how much you find. If the pan is full of metal shavings etc. it will also give you a forewarning that you have a trans issue.
If you're already getting messy and doing a full fluid replacement from the front of the vehicle, I don't see why wouldn't wouldn't drop the pan to get the gunk out. Otherwise the gunk is in there and will contaminate your new fluid. You'll be surprised how much you find. If the pan is full of metal shavings etc. it will also give you a forewarning that you have a trans issue.