For sure the blade needs to be parallel to the miter slot and the fence needs to be parallel to the blade/miter slot. I use the left miter slot because that's where I use miter gauges and jigs most of the time. You can set the outfeed end of the fence to toe out a little but don't use the fence on the other side of the blade if you do that, the fence would pinch the work as you cut. A benefit of having the fence exactly parallel is that you can use the fence on either side of the blade. I guess that matters more on a right tilt saw cutting bevels. My second thought would be to try a piece of decent quality plywood. Solid wood can move when cut, I've never had decent plywood move much at all. Decent plywood does not mean crappy CDX.