It really depends on your design. I have done ones with 2, 3, and 4 inches apart. The 2 inches does not give you the nice tiered step down look, as much as the 3 or 4 inches does. Also you have to take into account how many servings you will need. Give us more about what design you are wanting and maybe someone else can be of more help.
Depends on the design and number of servings you're trying to achieve. I don't hold with any rules pertaining to size. I just try to make 'em so they look right (ok...so they look right to me! 
)
Here's one that is all 2" diff: http://forum.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1280057
This one as a 2" diff and a 4" diff: http://forum.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1232880
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