Which type are we talking about?
Here on the DH8, our maintenance engineers request us to delay boarding until tire changes have been completed. The aircraft is hoisted up by a manual hydraulic pump, and it is easier when the aircraft is as empty as possible. The maximum load on the hoist seems to be a factor as well. And, finally, the engineers prefer not to work in plain sight of the passengers. Not because they are in the slightest way non-presentable or not doing their job right, but to ease the minds of the many passengers with a fear of flying, of course.