Alot depends on how you want the finished product to look.
Most classic Nintendo machines had the glass vertical, with the monitor tilted back at an extreme angle (Donkey Kong, Popeye, etc...).
Atari varied by cabinet from vertical right at the front glass (Vindicators), to vertical glass, monitor leaning back, (Asteroids), to layback monitors (Toobin').
Most of Midway's stuff has the monitor mounted near horizontally (Pacman, etc...).
Other companies just did whatever they thought looked good.
I'd pick a original cab design that you like, making sure it's one that fits the cab you are building, and use that method of mounting.
I personally like the Nintendo method, so that is what I used on my vertical-only cab, although I didn't lean the monitor back as far as they did.