A brief follow-up to my tolerance problem:
Today, I tried to remove the SlikStik Skirted knob from the Oscar Up/Down spinner. It wouldn't budge. This seemed strange to me, as it slid on fairly easily.
After a
lot of pulling and some WD-40, I finally got the knob off.

Why had it stuck? After looking closely at the DOT shaft, I noticed that the set screws had "dimpled" the shaft where they had contacted it. I hadn't set them that hard (I thought!), but they had apparently put small bumps into the shaft where they contacted it.
These "dimples" (holes + a ridge around the hole) thickened the shaft at those points, locking the knob onto the shaft.
This shouldn't be a problem when you've put the final knob on an installed DOT spinner, but it was a problem with all the knob-swapping I plan on doing. I sanded down the dimples, but I'm going to have to watch the set screws with future knobs.
Kevin