You have to have more than 2" difference. Meaning you can't use 12/10/8. If you measure the 10" from side to side, it's 10". But if you measure it point to point, it's 14" across, so it won't fit on a 12" cake if you stack the cakes diamond/square/diamond.
Measure the pans point to point (or stack them on your counter) to see what works best. If they are not ordering enough cake for the larger sizes, you might try talking them into setting the cakes "slightly askew", meaning turned slightly like a winding staircase. Here's a pic of what I mean (coz I'm not sure I'm explaining it right 
): http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1199535
No that won't work. You have to measure. Because you want some room for a border too.
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