For me, it changes according to needs. I like to be as low as I can go. I have one tree stand where the platform is about 3 feet off the ground. It allows me to see (and shoot) over the marsh grass and the tree trunk and bits of branches afford me great cover while doing so.
I tend to try so stay within the cover, and since I hunt a marsh area, my platform is typically 10 feet or less, however I have had a couple that needed to be higher, maybe as much as 22 feet. For me, that is rare.
I don't use height to counter my scent, I hunt the wind 100%.
Actually, I have been hunting from the ground in my RS ghillie more and more and having a surprising amount of success (success being deer within very easy range). I have let a lot of deer walk by the past few years.
ChuckC