I'm using a very fine flat blade. I tried spiral blades, but, while it's actually easier cutting corners and tight radiuses (radii ?) using a spiral blade, even the finest blade makes a quite bigger cut. I'm using the cutouts to align the pieces I'm glueing to the backer board and found that it's going out of whack because the spiral blade removed too much material.
Also, the spiral blade I used does a much rougher cut than the flat blade, which required me to do much more sanding to get a result that still didn't look quite as smooth.