My first comment is that for most bows you shouldn't need a 10yd pin. If you use your 20yd pin for a 10yd target (on level ground), you should hit around 2" or so high depending on your arrow speed. I would recommend using 20yd and 30yd pins, and a 40 pin if you comfortable at that distance. Your 20 will work just fine for anything under that distance, and will eliminate which pin to use when they're up close.
When shooting uphill or downhill, Gravity must also be considered. If you've ever ranged a target from a treestand or elevated position and fired an arrow based on that distance, you'll find that the arrow hits the target high. This is gravity coming into play. The height you climb a tree will certainly affect where the arrow hits. The higher you climb a tree and the steeper your shot angle, the higher your arrow will hit.
I typically climb around 20' up a tree and have found that I range the distance to a target from the base of my tree (on near level ground) and not the distance from me to the target. This will vary slightly depending on your elevation.
I pasted a link to some info that better describes arrow trajectory. Its definitely good reading to know where to aim in different situations. It also explains how your arrow travels up before it comes down...to help explain to some people why they're hitting tree branches that they didn't think were in the arrows trajectory path.
http://www.gandermountain.com/advice...0article.shtml
Good luck all.
Don