My data point(s) from last Friday:
3 segment itinerary, A B C. Cleared C at T-96. A showed R=4 for more than a week, but no upgrade by the time I checked-in at T-16. Called 1K line and agent basically said, "Let's see, R=4? Your upgrade should have cleared already. let me do it now." After he did, he asked if I wanted to SDC the B flight (which had F=1, R=0) to one an hour later as it was R=4 and he would immediately clear the upgrade.

So CPUs might not draw from R, but some agents seem to believe R needs to be > 0 for CPUs.