When my top whorl spindles get full, even with a notch, at times the fiber (okay, usually wool, grin!) will slip out of the notch and slip around the whorl a bit.
These days I always use an X wind-on -- the cop stays more stable, and builds up a roughly football shape. This lets me wind down to the bottom of the football and then come straight up from the bottom, through the notch and through the hook. That lets me spin a bit longer.
When I wind on the next length, I unwind a little bit so it's winding on the fattest part of the cop rather than lengthening the cop by winding new yarn on to the bottom each time.
If I'm trying to stuff even more on, I might through a half hitch around the hook. Yep, you heard me -- on my top whorl spindle, I'll put a half hitch on the hook. Then I can stuff on another few yards.
Now, if the spindle doesn't have a notch, you could add one (ouch, I hear you say!). Okay,if you don't want to do that, another way to add a "yarn stop" is to start spinning (when the spindle is empty) by hooking the hook into the fiber supply, spinning a length, and then with that length on the hook, wind down, around the shaft, and then back up through the hook -- the first few inches then act as a natural yarn stop for the new yarn to rest against, stopping its tendency to unwind when the spindle is full.
Someone once suggested that I add a velcro dot (the side that things catch on) on the side, top, or underside of the whorl to hold onto the yarn as a notch-substitute (haven't tried that one, though).
(based on a post by me on spindlers, 11June07)