I just spent two quality hours with my table saw. I'd known for a while that the saw needed tuning, but I'd been putting it off. Yesterday's frustrating session, however, was the last straw. I'd been checking the saw for true for three years using the blade and a square, but it doesn't really work well -- the blade is too small and has the carbide teeth that throw things off. It always looked "close enough" but of course, in fine woodworking there's no such thing. So I hauled out the MasterGage MasterPlate and SuperBar that had been sitting in a drawer for three years (!) and set to work.
There were three problems: The table was slightly askew, and the stops for the blade and for the miter (or mitre) gauge were all slightly off. Basically the only part of the saw that was right was the rip fence. So I replaced the blade with the MasterPlate, loosened the top of the saw (~80-100lb piece of cast iron) and set to work getting the SuperBar to read less than 1/100th of an inch difference across the length of the plate.
Once that was done, I set the miter gauge to 90 degrees and checked it against the plate and then adjusted the screws until there were no gaps. Repeat for 45 degrees. Next, tilt the blade to 90 degrees and dork with the set screws until it was really 90. Repeat for 45 degrees.
Run some test lumber through and verify all the angles coming off the saw are correct -- that two 45s make a 90 or a 180, and that all rips are parallel to each other and square to the miter gauge.
If you own a table saw and don't have a MasterPlate and SuperBar, borrow one. You'll need them every year at least, and they make a nearly impossible job simple.