ORIGINAL: blw
I always pull the backplate as a minimum before running any engine, new or used. The heads of 2 strokes come off and usually the carb too.
Its cheap insurance to look them over. I've cleaned enough metal shavings out in the past. Not good.
Most of my rc car engines have metal caged bearings, now that I think about it. Those engines turn 33,000rpm+, and the bearings never explode. What's so different with airplane engines?