One main function of Skate Sharpening Is to Produce Even Edges On Your Blades.
The sharpener must be able to check the finished skate to determine if the edges are even.  If the edges do not come out even, he/she must be able to determine which edge is high and by how much.
Refer to
Figure 2.  An old method to check for even edges is to place a coin flat onto the bottom of your blade (Figure 2a). If the edges are even, the coin will form a 90 degree angle to the side of the skate blade. There are two problems with this method, (1) The coin tends to easily fall off the blade, and (2) the coin does not extend far enough out either side to magnify any error.
In Figure 2b, a 6" flat object is used.  We now have 3" of material sticking out both sides to magnify errors, making error recognition  easier.  
Figure 3 displays how the skate clamp height or angle settings can effect even/uneven edges during the sharpening process.
Tip - Use a plastic credit card (drivers license) and hold it in place with a finger tip.
After sharpening, every skate should be checked for even edges.
Note: Drawings are exaggerated for effect.
The information above was originally presented by Wissota Mfg, at www.wissota.com