Well I used to break in on the plane but the last one I break in on the test stand and test stand is not a big deal, you can buy a birch ply for $2-3 and mount the engine mounts you are using in the plane with blind nuts.

You don't have to worry about holding the plane down, mess on the plane, and I think safer assuming that you mount good on the stand especialy with big engines, it is a lot convinient to do it. and you have both of your hands to do adjustments not holding down anything.

the next engine small or big, I want to break in on stand as it is just couple of minute extra work but convinient