I have to call BS too. I have heard that for years but I dont believe it is true. The only problem I could see is if you had some trash tucked away in a corner of your valve body, flushing it could dislodge it and get stuck in one of the passages. I say go ahead and change the fluid and be done with it.
2000 Silverado, RCSB, 2wd w/ a few mods.
Best 1/4 ET: 12.09@111.22 (old 5.3L H/C/N2O setup)
WRE 370 and Kenne Bell 2.8L installed and breaking trannys.
2001 Tahoe LT 4wd
Stock, daily driver and tow vehicle.