A dremel type grinder would work for both. You'd want to mask off the case so you know when you're chewing too far into the top case. A guide or jig to hold the grinding wheel stationary would be better since you'd be less likely to go too far into the battery for example on the Polycarbonite model.
If you are dead set against changing your ergonomics so your wrists never know whether the edge is sharp or beveled, I would get help from someone in a machine shop that has experience with grinding. They will have practiced on many other items and likely have far better technique. The tools to both cut and polish polycarbonate and aluminum are different, but knowing how to clamp down the mac and use a metal file properly would come in handy - especially if the shop has the tools and clamps needed and you only have to supply the patient and perhaps the finishing abrasive and social lubricant to get the job done.
If you are going to go the DIY route, emery board might be better and less risky and you can control the amount of material that is removed over time. Start with that, even on the aluminum and work up to a metal file if needed.
You'll want a fine emery board to polish things either way, but do try to get a scrap case to test on before you start (or at least get a quote to replace the part should you like the modified case worse than the new edge.)
