The guy at the store is an idiot. Don't listen to him. The purpose of breaking in an engine is to get it to OPERATING TEMPERATURE so that surfaces mate against each other.

If your engine is ABC (non-ringed) running it at idle and low temperature conditions (less than 1/2 throttle) may cause excessive wear of the cylinder taper (causing loss of compression and premature engine death).

If your engine is ringed, running it in at low temps will not harm it but will not break it in either.

Use the search function on this website and you'll find many responses on how to break in an engine. Another good bet is to open the little booklet that came with your engine and read it. It probably gives a recommended engine break in procedure.