I have 1 year
experience with top bar hives and thousands of hours of spending time on biobees and beesource forum
Im going with the top entrance on all my hive so condensation is not an issue in winter. I use one entrance hole only which is 30mm so the
bees can easily decide how much of hive atmosphere can be let out during the summer. Varroa doesnt prefer high temperature. I dont know if this is good dry climate but in my damp one it might help reduce the condensation levels keeping the hive dry. We have lots of
water around here in form of lakes and ponds while in dry climate condensation is a great resource for the
bees so bottom entrance is a better choice I think.
When it coems to placement I try to drill holes on one side of the hive to encourage bees to build in one dirrection. On some hives I have top entrance at the end and on some on the end of the sides;
http://cheguebeeapiary.blogspot.se/2013/02/landing-boards-and-new-apiary-location.html
Phil Chandler uses tree 20mm holes in the middle of the side walls (bottom entrance). In his design two follower boards are needed in mine only one.
Both seem to work fine.