Assuming a 100% factory truck, you cannot hurt a factory 4L60E or 4L80E moving the shifter into any forward gear at any speed. You can pull the shifter into 1st at 100 mph if you want. The transmission will not downshift until the vehicle speed drops enough for it to downshift. Downshifting does not hurt a thing, I do it all the time slowing down especially towing.
If I put the van in 1st at 80 mph, it will downshift to 3rd, then at 70 mph it will drop to 2nd, then at 40 mph it goes into 1st. My maximum downshift speeds are a bit higher than GM had programmed for stock 4L80E van application. I do not have to contend with drive by noise testing like they do.
It is actually easier on a 700r4, 4L60E and 4L80E to start off with the shifter in 3rd gear. It locks in an additional clutch pack that helps keep the sprag clutch intact. Especially important when towing.