I think it may depend on what you intend to use it for. The guys I've seen that use the disc for grinding bevels all had the disc vertical. If you're going to add a tool rest and use it for profiling or squaring edges, vertical also makes sense. I wanted mine strictly for flattening so I mounted the disc horizontal with a VS rev. motor. I've also tried it for fine sanding and polishing of blade parts. My mirror finish still sucks!
It is simple enough to make the disk grinder convertable so you actualy wind up with two tools for the price of one. Then you can decide how you want to use it each time.
My disc grinder is reversible and gets used a lot. I'm going to look at Wayne Coe's video as soon as I post this to check out his vertical/horizontal setup
After having both thru the years (and 3 vari reversing) in my shop right now I'll put in a plug
for the vertical with a tool rest. For me I can see what I'm doing a lot better esp. when
flat grinding a blade.
Ken