It's a good indicator, JB, but not infallible. Some seeds are naturally lighter and may not be able to absorb enough water to sink them, others have hairs that trap air and cause them to float. I've had plants from very large seeds that I discarded because they never sank come roaring out of whatever I discarded them in, defying the rule about floaters and sinkers.
If the only seeds I have of something I really want to grow float, I don't let that stop me, but I adjust my expectations accordingly.
