Whether the spool is horizontal or vertical is less of an issue than whether the thread is unwinding off the end of the spool or straight off sideways.  And in fact, which way it was wound on the spool in the first place.  I and a lot of others use separate vertical thread stands to give the thread a chance to relax the twist.  

I have been looking at the M17 manual and by using the spool pins and the thread guide bar, you are unwinding the thread in the same way as a horizontal thread holder.  I like this new configuration because it is compatible with straight wound thread, without having to use an adapter.

Here is a link from Superior Threads Education area that talks about feeding thread off of spools. 

-Pixey