The positioning depends greatly upon the
lens and the relationship to the audience, as well as
practical considerations such as how you can mount the projector.
Most very short throw lenses (<1.0:1) rarely have any keystone correction capability and defeat any lens shift functionality, thus almost always requiring that the projector be on the centerline of the image with those lenses. Most other lenses will have the projector ideally located near the top or bottom of the image.
If your audience area is sloped up away from the screen then consider putting the projector towards the bottom of the screen. If the audience area is flat or below the screen, then having the projector towards the top of the image may work better. However, the projector location is often dictated more by practical considerations than it is by technical ideals.
Somewhere in the projector's on-screen menus is probably a projector configuration option. Typical selections would be front projection tabletop, front projection ceiling, rear projection tabletop and rear projection ceiling. If you can't find this and don't have the projector manual, you can usually find manuals somewhere on the manufacturer's web site, typically either with the product data or under technical support.