In my experience, it doesn't really matter. If the wisteria's happy (and it isn't, always, something I'm doing some research on, if it'll help folks who experience 'wisteria melt') then it will put out many roots, whether in the ground or floating.
I tend to float mine before I plant it, just because it's pretty and the bettas really like it. Then when the roots get stupidly long, I plant it.
And it doesn't have to be planted straight up and down at that stage. It sends out roots at all its 'joints' so it can be planted longways, to let lots of smaller plants spring up, too.