I have been doing the same thing carving lithophanes, By lying to the software about the bit size you can get a slightly different effect in the the carving that manipulating the stepover alone cannot do. You just have to visualize how the difference in tools will actualy carve the material, and choose what you tell the software accordingly. I first did this on a litho that did not carve well with an engraving bit... Having nothing to lose, I ran the toolpath again using a different bit, and was pleased with the result. Ever since I have been slowly figuring out how to use this to my advantage. With my lithos, I have found using a slightly larger bit than specified in the software still gives good detail, but slightly smooths the carving which many times gives a better machining job on tricky materials. There is a loss of real crisp features, but if the material cannot hold that detail, and have a good finish ther is no real gain in the finished project.