Many times expression blend will add minheight to my grid rows without me asking it to. Is there a way to disable this?
Minheight gets added to rows (and Minwidth to Columns) when you use the Lock icon on the Artboard to change to the "Auto" sizing method. The answer is to either not use Auto sized rows and columns or to just fix it as you go.
You are right, this is totally annoying. It's even worse when you don't realize that's what's happening: then the Grid doesn't work as advertised and you can't figure out why without digging into the XAML. It's been suggested to the team that this behavior be changed for a future release.