Thanks! I do have a hankering for some 5-axis toolpath generation algorithm work at some point down the road, but PixelCNC is not the project that I will be tackling 5-axes with.
PixelCNC's primary focus is on loading 2D images as input (although v1.30a will be released today which includes 3D model import functionality, but that just generates a 2D image from the 3D model geometry). As a consequence of the 2D image paradigm the complexity of a project never necessitates more than 3-axes of cutter articulation - with the exception of projects that may have deep spots where a tool is not long enough to reach a cut from a vertical approach, and the chuck will crash into the material.
Otherwise, as long as the cutting tool is long enough to prevent the spindle from crashing into the material a 3-axis will always be able to do anything PixelCNC is capable of generating simply because every surface of a PixelCNC project is accessible from directly above it. There are never overhangs or undercuts, etc.. This is just the nature of 2D heightfields, which is effectively PixelCNC's core premise.
In either case, there's no reason you can't run 3-axis programs on a 5-axis CNC! :)