It depends on your location, the satellite(s) you're trying to receive, and the height of the tree(s) in question.
I have a 20' tree that is about 15' or so in front of my 10' and 4' dishes, but since my lattitude is around 30 degrees and the tree is due south it doesn't really interfere with my reception. The 10' dish is about 8 feet off the ground and my 4' is about 6 feet off the ground.
You can try using dish pointer...it has a tool that allows you to judge whether an obsticle's height will pose a problem.
Good advice, what I did to decide how high to go with my dish was got a cheep angle finder, found the angle for the sats at my location then used the angle finder to sight off of, added more space for future growth and put in the dish.