Depends on the plant. In general rosette plants (like Amazon swords, Aponogetons, or Crypts) get a lot of their nutrients from the substrate, and they don't like their roots disturbed. These plants won't really grow floating, either. And, the purpose of "good" substrates like flourite is their property to make nutrients available to plants, not to actually give them nutrients.
As for stem plants, these are the ones that, well, grow as individual stems. They may branch, but all the way at the bottom only one stem enters the substrate. In general they can get a lot of nutrients out of the water, which is why they're generally good water purifiers. They can grow floating as well. They grow really long, so they have to be trimmed often. If you cut off the tops, they look ugly, but may branch more, depending on the plant. The best thing to do is pull up the whole stem, cut off the bottom and replant the top. It will grow roots and anchor itself within a few days. You will also notice that if you leave stem plants rooted for a few weeks or months, they develop very extensive root systems. They don't mind being pulled up though. The roots along the stem are totally normal, that's just the way some plants are.