If you really want to make sure the knobs stay put, get setscrews (either with knurled knobs that screw in by hand, or better, slotted for a Phillips screwdriver). Also get a metal punch set (looks like a chisel, but pointed), often used for starting drill bits in metal.
Slide the knob onto the shaft without the setscrew, and mark where the hole lines up. Then take the punch set, and dimple the shaft where the mark is. Then, when you tighten the setscrew, it will tighten against the dimple, and should stay tight longer than it will against a smooth shaft where it could slip.
I thought getting old would take longer!