A deer isn't going to know the difference between 12 ft. and 15 ft. It's the guy setting up in the stand that's going to spook him. You can hunt 10 ft., 5 ft. or whatever you want. If the wind is in your favor and you don't get caught moving you'll win. However the lower you keep getting the closer you are to being in the line of site of the deer.

I killed a deer once with a bow and my feet was only five feet over his back, so it can be done. I like 15 to 18 ft. myself but it's whatever you feel comfortable with. I have hunted stands as high as 30 ft. but as I get older I feel better alittle lower.