I just bought a 1999 f150 with the 4r70w transmission. I have no idea if its ever been flushed and the tranny fluid is low and smells a little burnt but is still a red color. It shifts fine I just wonder if I should do a flush or just add some
Opinions vary on the value of flushing the system. Some people think the friction material has broken off the fiber plates and is circulating with the fluid. You don't want to remove that material with a flush since it's needed to make the plates lock up. I'd argue that when friction material is circulating, it's either going to be trapped in the filter or it's going to chew up the lip seal in the clutch packs.
I suspect the people who say you don't want to remove the friction material have had bad experiences with flushes on high-mileage vehicles. There's nothing wrong with doing a flush as long as there are no problems or symptoms being chased. If there is, the flush won't magically fix wear or damage that has already occurred. When that wear continues to the point of slipping clutch packs, it's the wear that was the cause but it's the flushing procedure that gets the blame.
My preference is for a filter and fluid change about every 36,000 miles or when the manufacturer recommends. That replaces about half of the fluid. There's a separate flushing procedure for just the transmission cooler when the transmission is replaced or rebuilt. That is required by most manufacturers to clean out any debris or friction material so it doesn't contaminate the new transmission.