One reason for double-tapping the AR is that it's sometimes required in order to ensure the target's down-and-out. It can be a commonality of technique thing with pistols as well, for the same reason.
Even if the target's wearing armor, a solid or even peripheral hit from a .30-'06 or 7.62 NATO will usually put 'em down, mostly for good.
Another issue is target-re-acquisition/recoil. Unlike the AR, the .30 cal battle rifles have significant recoil. Given that fact, it's usually more profitable to transition to another target than to re-align on one that's already probably down for the count. If you insist on double-tapping a Garand or M1A, a [b]good[/b] compensator is a help.
Personally, I'd make sure the first shot was a solid one, forego the comp, and add a flash suppressor (where allowed by law, of course) [:D].
Of course, YMMV