BOY!!. That is TOTAL BULLSH*T on Best Buy's install service part.
Unless it was TOTALLY NECESSARY for them to put it on your dashboard, that is total crap for a pro install.
I would take it back, and unless they accomodate you(which I doubt, but I would hold out for a gift card or something) they should definitely place it on the roof.
Unless of course you are leaving out a VALUABLE piece of info here. Like you drive a convertible with a cloth roof or the like(the magnet on the bottom won't "stick" to the roof"). If that is the case, ignore the above rant. If not, get it placed on the roof of the car.
Anything outside of that on a PROFESSIONAL install is ludicrous.
About the dropouts with trees and overpasses, that is the norm. Especially if it is installed on your dash, and all you achieve is 1-2 bars MAX on signal reception(especially with 1 bar, as dropouts would be frequent.
If you can install it on the roof, then dropouts would almost completely disappear(still a few occasions, but no more then a "blip"). But would still be VERY RARE.