Thermals fall in the evening and rise in the morning. Which means, If you are hunting the mornings and you are 20 feet up with little or no wind, the deer are less likely to smell you as your scent is going up. On the other hand, in the evening if you have little or no wind try to imagine this 50 yard invisible cloud of scent in a circle around the base of your stand. It's kind of like sitting in lowland and you know how the light fog sets in around you? Kind of the same thing. Hope this helps.
That said, I hunt a minimum of 18' and the majority of my stands are 20-22'.