I may be reading to much into your question but you DON'T want to seal the floor (per se) UNLESS you're want a 'clear' floor. There are products called Sand Seal that seal the wood and PREVENT anything (stain included) from getting absorbed into the wood. Flooring stain, by design, has a sealer already in it. Soooo, you DON'T need to 'apply' anything more other than the poly/finish. See what I mean?
I just thought I'd add this. 'jb' did say to just, "Stain and three coats of poly will seal it up ..." I just want to make sure you don't do anything 'beyond' those instructions.