5 degrees works fine with a 14-15 pound engine. A Wren SS is a good power match for a Falcon 120. I found that mine still had a tendency to climb at higher throttle openings with 5 degrees. I changed to 6 degrees and now it's just right.
It has good acceleration and no trouble rotating. Those who have trouble rotating most likely do not have a couple degrees of nose up attitude as the plane sits on level ground.
Go with 6 degrees downthrust and 1 or 2 degrees of nose up attitude and you'll have smooth take-offs and no tendancy to climb.
Here's a video of mine in action. Not the best video since the camera doesn't have a zoom lens.
http://www.youtube.com/watch?v=wy7hnNOrSlY
Joe