I haven't read everyone's answers -- but here's my view: It's according to table space and size of the project. I just trimmed a 108" wide backing and had to cut from right to left (horizontal?). Usually I cut vertical -- from my belly to the top of the table.
If you go to
www.youtube.com and type in "how to use a rotary cutter", you'll find a ton of videos on how to cut.
Have fun!