This rule of thumb comes from just my experiences in the past couple of weeks. It applies to a number of things, actually, but the first things that come to mind are your wood measurements, and your step-by-step guides.
I bought a book put out by the publishers of a popular woodworking magazine, in which they show a picture of a bevel cut being done on a table saw with the blade tilted *toward* the rip fence. DO NOT DO THIS! I’ve read in several other places, and heard from experienced woodworkers’ first hand experience that this is absolutely the *wrong* way to set up a bevel cut on a table saw, not because of accuracy issues, but from a safety perspective. If you perform a cut this way, you *will* have a piece of wood flung at you at high speed sooner than later. When learning the basics, USE MORE THAN ONE SOURCE to learn how to perform the basics like bevel cuts. A terriffic source is google groups. Do a google search for something like “bevel cuts on a table saw”, then click on the “Groups” link on the top of the Google page. You’ll see results from woodworking usenet groups where people will share their experiences. Good stuff. Another good source is Wood magazine’s online forums. Look around, I’m sure there are plenty of other good sources of information.
Besides educational sources, there are informational sources on your wood. Take more than one measurement for everything you do. If you can figure out how to take three or four measurements to “prove” that something is absolutely correct, do it. If you have tools around that can aid you in this (like a square or combination square or protractor or caliper or bevel gauge), use them as more proof that what you’re looking at is how it’s supposed to be. This is how I learned my earlier lesson about not trusting wood to be milled perfectly square. I measured out some cuts, then realized they were off because the boards weren’t perfectly cut. I measured with two different rulers, used a combination square, and a bevel gauge to figure out that my board’s edges were the problem. Once I drew my cuts without using the edges (ie, I drew all four sides of the cuts), everything was fine, and I knew that because I used two rulers and a bevel gauge to prove it. I measured the diagonals of the square I drew, and measured the length and width at various places along the sides of my cut. When I was confident it was perfect, I cut it out (and screwed it up, but that’s another story – at least my drawings were good!)