I bench run engines for a couple reasons. First, it's easy to get to everything and make adjustments. I've found that once an engine is close to being adjusted right, it doesn't matter how you mount it, the adjustments don't change much. The settings do change a little due to different fuel tank location, distance, etc. but if the engine is adjusted properly on the bench it's only a couple clicks of the needles to get it set in the plane.

Also, an engine has to be at least a little broken in before a reliable idle can be set. I almost never land dead stick unless I run out of fuel because I spend a good bit of time getting the idle dialed in.

Second, I want to make sure the engine works before I spend all the time to mount it in a plane. Even if I put a new engine in the plane I'm going to run a tank of fuel through it before I fly. I'd rather use the bench than have the engine throwing oil and dirt all over the plane.

Old engines took forever to break in but that's not true any more. If people are still running a gallon of fuel through an engine then they're living in the past or running old engines. I run a tank or two and then it goes in the plane.