I've heard that as well, but really question the logic of it. This was somewhat true in the older days, but I dont think it applies to the 700R4.
Also, in terms of filter changes, you will have to drain and drop the pan regardless- thats good for about 6 quarts or so of fluid. I'd install a drain plug in the pan to help you out for the next drain. (you'll see how much of a POS GM was to not put in a drain plug).
IIRC, overall fluid capacity is over 8 quarts.. I think its in the double digits. You probably wont use over 2 gallons though. The rest of the fluid is in the lines and torque converter, and in my opinion, not worth the effort to get out. You can actually get away with Partial changes like this, which works pretty well. Every year, drain fluid, drop pan, change filter.
i've added in an additional inline filter from permacool. The stock filter is around 25-40 dollars, depending on where you go. The permacool filter is any regular ford FL1A filter.
Stock filter is acutally pretty useless- only captures big chunks. Its filtration capacity is maybe 50-100 microns. the permacool setup reduces particles in the 10 micron range. However, if you truly want the best setup, you'll have to go bypass which will remove particles smaller than 2 micron.