I see frequent references to a sliding table panel saw being set up with the fence square to the blade.
A better way to set the fence up for a truly square cut, is to set the fence square to the direction of travel. If your blade is in perfect alignment, ie, parallel to the travel, it amounts to the same thing, but setting the fence to the table travel eliminates compound errors when the table rails might be a bit out.
Think of the blade as a pin that sticks out from the table and the work runs past it. It doesn't matter what angle the work runs past the cutting point, as long as the good edge that is placed against the fence runs perpendicular to the travel, you will get a square cut.
The point about the table rails being a touch out is valid, an ex-Wadkin engineer told me some years ago that they set panel saws up so that 1) the sliding table runs very slightly away from the blade, so that the upcoming teeth on the back of the blade clear the cut and 2) the rip fence is also set so that a rip cut leaves clearance at the back of the blade.
A square in the table slots is more reliable than a square against the blade. Check the squareness using the 5-cut method.