What ever safe height feels comfortable to you --> is the answer to the high end.

The second question I usually ask myself is what tree conceals me the best.
And will / may dictate the height based on that day's hunt with the wind direction.
I usually try and pick a few trees for the different winds for that matter.
If I have a choice a nice hemlock would be it. (slightly off subject).
Three main reasons for a hemlock if I have the choice (in no particular order)
1) less steps - as you can usually climb it like a monkey
2) As mentioned - concealment
3) the natural odor - while sitting I'll pull some needles off a near by branch and I'll even chew a bit of them to camoflague my breath (allright - I'm kidding about pulling needles off) (but chewing them makes a great gum!)