What are you looking to do with them? Are you going to try to go in any direction, try to turn better, or some other aspect? Most of the omni wheels that I have seen have allowed the robot to go in any direction. But I have seen quite a few that were used to make the robot turn a lot easier by putting them on the four outside wheels on a 6 wheel drive system. It all depends on what you wish to do with them.
I just posted the image for reference.