The bees don't care where the entrance is. Here are the reasons I went to top entrances:
1) Skunks. I never had problems with them before in other places, but here they have been a real problem. Sometimes the possums too. No bottom entrance and just the top entrance stops them altogether.
2) Mice. These are only a problem in the winter and again, when I had bees in other locations they weren't as bad as here. But no bottom entrance stops them also.
3) Tall grass. The grass gets tall and blocks the bottom of the hive. That's not a problem with a top entrance so I don't have to mow.
4) Deep snow. In the winter here the snow gets deep and blocks the bottom entrances. Without a top entrance the bees can get stuck so they can't fly on a warm day witn snow on the ground.
5) Dead bees in the winter. Dead bees sometimes pile up in the winter and block a bottom entrance. This is not a problem with a top entrance.
6) Ventilation. In the winter a top entrnace helps with condensation on the lid and in the summer a bottom and top entrance make a world of difference in production (drying nectar goes faster) and less swarming (bees that aren't all clustered outside in the heat seem less prone to swarm). I would not use it as your primary swarm control, but it does help.
I just prop up a lid to make a top entrance. Shingle shims work well. There are pictures of a lot of them on my web site.www.bushfarms.com