On the FineWoodworking Website Andy Rae demonstrates (in video) how to put a new edge on a plane iron. As part of his instruction, he reviews how to true up a grinding wheel.
According to him, the diamond 'dressers' take off too much of the wheel surface too quickly, create a mess and make it difficult to put a square face on the wheel. He prefers the little carborundum sticks sold at hardware stores for the purpose.
I don't sharpen using a grinder, but occasionally have to put a new square edge on a tool.
Gary Curtis