A lot of this depends on your previous experiences working with small tolerances, and your ability to SEE small stuff. At the age of 53, I resort regularly to an Optivisor with 2X and 2.75X lenses, and a 10X jeweler's loupe. I mean, you can pretty much cut everything bigger than the B string slot with varying thicknesses of sandpaper folded over a nail file... but only if you have a very clear picture of why you're doing what you're doing, and you know your tools. (I personally use abrasive wet/dry sandpaper to do a lot of stuff like leveling frets that others use files or diamond files to do - I just know that stuff pretty well, and consistency is a result of familiarity with your methods). If you're working to the highest professional standard, when you press the strings down at the second fret, the high E should clear the first fret by about .005" - half it's diameter. The low E should clear by about .010" or so. LMII sells something called the Zona saw:
http://www.lmii.com/CartTwo/thirdproducts.asp?CategoryName=Knives+%26+Saws&NameProdHeader=Zona%99+Saw
It comes with three different blades of usefully different widths. Also the smallest saw blade in an X-Acto set is usable for the .010" slot. LMII also has an extremely useful file called a "pippen" file:
http://www.lmii.com/CartTwo/thirdproducts.asp?CategoryName=Rasps%2FFiles&NameProdHeader=Pippin+File
A pippen file has a continuously-varying width on both edges from maybe .015" out to bass string low E widths.
I highly highly recommend to anyone laying in to this stuff, that you first buy Dan Erlewine's "Guitar Player Repair Guide." And read every section that applies to what you want, it's a good way to assess your comfort level. Even if you still pay people to do some of this, that book will still save you thousands of dollars over your lifetime, because you'll recognize if a tech guy is blowing smoke. Erlewine shows the method of using stacked feeler gauge blades that makes it impossible to cut nut slots too deep. And yes, the slots can be wider than the strings, as long as they angle back toward the tuners so that the string breaks over a single point of vibration. I change string gauges a lot, and once I've set up for 10-46, I may only have to adjust a bit to lay on some 12-56's for slide - and I can
usually go back to the 10's with no problem.
I won't even
mention the dreaded toilet paper/superglue trick, because it's so wrong and bad - but it works great if you goink a single slot. Well OK - you cut a piece of TP as small as you can get it to fit a doubled piece inside the nut slot, then you drip a single drop of superglue on it. The TP isn't structural, it just hold the glue in place. Then you refile the slot. Luthiers do this, but you're not supposed to know about it.
It is useful to know that jewelers, gunsmiths and watch repairmen use many of the exact same tools as guitar techs, only they don't pay three times as much for them..... I have a few files I bought as "nut files", but I just grab all 15 or however many I own, and the bespoke nut files don't get used much. One very useful file to track down is a small flat triangle shape with teeth on only the widest side, because it will widen nut slots without deepening them (a hazard of the sandpaper-wrapped-nailfile that must be noidally watched).