Depends on the quality of the solder joint. Poor quality solder joints do not survive mechanical shock. If you are fighting a GND pin that sinks a lot of heat, using leadfree solder and you aren't that skilled...don't trim that lead flush with the PCB. Otherwise, if you are sure that the solder has wicked into the hole, trim away.

So if someone is telling you not to trim the lead...I'll let you draw your own conclusion.