Don't really know of anyone who flushes/changes out the power steering fluid - short of it getting contaminated, or unless you have a leak, pretty well everyone leaves it alone.
I've never completely flushed mine, but what I do is, I'll suction out as much of the old fluid as I can, add some new fluid, drive for a couple of days, repeat the process.
While I realize that I'm not changing the fluid (just diluting the old with the new), it's easier than flushing out the system, and I get the satisfaction of knowing the the fluid is relatively new and clean. I do this every couple of years.
Don't have any unequivocal proof that this "helps" in any real qualitative way, but I don't mind doing it, and it certainly can't hurt.
BTW, over 300,000 km (over 200,000 miles) on the truck, all original, and the power steering pump doesn't even have the "squeal" that Ford PS pumps are known for.
'03 Ranger Supercab, Flareside, 4.0 liter V6 SOHC, auto, 4 x 4 w/FX4 Off-Road Package (Level I)