I've heard that soldiers who served in Vietnam frequently downloaded their M16 rifle magazines by a few rounds (e.g., only loaded 17 rounds rather than a full 20 rounds) to promote reliable feeding.  Is that still a good practice?

It proved to be with dirty worn out mags and dirty guns.

Its not so much of an issue for civilians who have equipment in better condition for the most part.  Test your guns and see if they work consistantly with a full magazine.  Some guns will, some wont depending on tolerances.

The biggest reason I load my 30's with 28 rounds is it allows me to do a tactical reload with the bolt foward on a chambered round.

Interesting.  I hadn't considered that tactical reload scenario.  Makes sense.

I have one AR15 in particular (an older Colt SP1) that's consistently reliable with fully loaded 30 round mil spec magazines, but refuses to feed the first two rounds of fully loaded 20 round mil spec magazines.  The 20 rounders work fine with all of my A2 rifles, but not with the SP1.  I've concluded the magazines are OK. Like you indicated, I'm thinking the tolerances or, perhaps, the geometry of the SP1 must be just a little bit different to cause the malfunctions.

My carbine will "sometimes" not load with a
full mag(USGI 30).  The springs are new though.  So, when I do matches I down load to 29 or 28.

Joe