DLink and other OEM Mfrs don't give much information regarding. Again, as stated, Omni-directional is just that. Really is not dependent upon a certain angle or position of the antenna. Now relation to the router placement to the PCs is obviously as you would want a central placement of the router in a building so that the broadcasting signals emanating from the router can reach most PCs with in the range of the 360 degree signal path.
Now if the antenna and signal were in a directional setup, yes, angle and positioning of the antenna would be critical for reception, such as Microwave and Satellite and some analog communication.
Basically, just aim the antenna tips upwards from the router if you have it siting flat on its feet. If you have it on the vertical stand then move them so that they are pointed upwards as best you can.
FYI, the best signal mode to use is G mode only at 2.4Ghz if you don't have any N supported devices.