I've polled a few knowledgeable folks and a majority seem to prefer using a 19" tube versus a 21" in a 20" tire. Apparently, stretching a smaller tube over a larger rim might me safer than stuffing an oversized tube into a smaller tire. The concern is that possibility of folds could spell disaster. I had a look at the Coker Tire site and they have it both ways depending on the tire width. Wider, old-car width tubes sold by Coker are spec'd for 20/21" and the narrower tubes for motorbikes are 19/20"...but not a one just for 20"!
My choice will be the 19" tube for my 20" TR5 front wheels.