It’s easy to confuse some of this terminology. There are stopper knots, also known as backup knots, that would be tied at the figure-8 knot to ensure that it didn’t come untied, then there are stopper knots at the end off the rappel rope so you don’t go zipping off of it on the rappel.
If you’re referring to a stopper knot at the end of the rope so that you don’t rappel off, the I would say no. Just make sure that you can see the rope laying on the ground before you rappel down. I started off with a stopper knot at the end but untied it the first time I had to pull my rope down over a branch so that the rope could pass through the quick link, just never retied it since I usually only climb to around 20’ high and my rope is 36’.
I prefer the scaffold knot over the figure-8 knot to hold the quick link since it keeps it held tight and keeps it from rotating. If tied correctly with a decent tag end sticking out and a thick enough quick link the concern of pulling the quick link up into the knot is pretty much negated. That concern would be pretty much the main reason I can see for going with the figure-8 knot instead.