Even if the blade is a bit dull, if you're getting substantial wander, I would suspect the guides before the blade. I've used dull blades with good guides and I got straight cuts, they were just painfully slow. There should be very little side to side play in the guides (if you can fit a sheet of paper on each side, that's more than enough clearance) and the rear bearing should contact the back of the blade as soon as you start to feed the stock (~1/64" clearance when running free). I suffered with a lot of wander until I learned how to set up the guides on my saw properly.

BTW, with many saws, it's necessary to readjust the guides if you make substantial changes in the stock thickness you're cutting. The better saws are precisely made and don't suffer from this, but mine both require rear bearing adjustments depending on stock thickness. One of these days, I'll buy a Laguna and that'll be the end of it.

I've used blades with a small amount of set and with large amount of set and have found that the latter wander more, since they make a wide cut and are easily turned in their own kerf. The wide tooth set make them adept at removng material from the kerf walls with minimal side pressure. They probably work great if your band saw has a good fence system, but mine doesn't. I'm currently using a wide set blade, but I'll be going back to a narrower one once it's worn out.

I agree with Nick about blade tension, Make sure the blade is tight.