I use round balsa, I've got jig which has a hole the same size as the balsa, the jig has a smaller hole drilled through the centre. To use I push the balsa into the jig and the feed in the wire stem through the small hole, gives you a perfectly aligned stem.
Another tip, to get a a good smooth base for the float, mix talc powder with dope, it will fill the grain, two coats are normally all thats required.