In general, if you stagger the heights of the cameras it helps with calibration. Placing one or two a bit higher, like 8 to 10 feet, will give you better overhead coverage which can be helpful if the character is lying down. This should work well most of the time.
The idea is to give iPi Studio as much visual information about the performer as possible within the scope of 4 to 6 cameras. I also think it helps if there is some visual overlap between the cameras.
In special situations, the type of motion you're capturing may require a different arrangement. In these situations, you should experiment--i.e., place some cameras very low or very high, or even placing them closer together (i.e., less than 360 degrees coverage.) But I don't think you need to worry about this unless your performer is doing something unusual and Studio is having trouble tracking the data.
Here's the Wiki entry on the topic:
http://wiki.ipisoft.com/User_Guide_for_ ... figurationScroll down to Six Camera Configuration.
Hope this helps.
G.