I always take 2 passes... One to cut the tire to within 2mm of my final diameter, and the next to finish it off. I use the BSR carbide bit, which is by far and away the best bit I have ever seen. It can easily produce a smooth race-able tire with one-pass, but I always feel more comfortable doing 2.
Not to mention it puts less strain on the truer. I have a Hudy auto that I purchased when they were first released in ~2001-2002, and it is still going strong. Although it also has a lot to do with the durometer and type of foam. One-passing Pink rears is nothing... One-passing WGT Lilacs is another story.