As you mentioned in your question, using a crosscut sled is an easy and reliable method of making perfect 90 degree cuts on a table saw.
If you don't have a sled but your blade is properly aligned parallel to the miter slots, you can use a miter gauge. Most saws include a miter gauge, although some are better than others at locking in their settings without wobbling during a cut.
It's common practice to screw a longer wooden fence onto a miter gauge using the slots on either side of the miter gauge's face, as shown below. Some commercial miter gauges include aluminum fences that serve the same purpose--namely, providing better support for your workpiece close to the blade.
Since it was my first sled I was allowing for some irregularity
Your sled itself doesn't need to be any particular shape, and the base doesn't even need to be square with the rails. It only needs to have a back fence which is adjusted perpendicular to the blade (in the case of a crosscut sled) or whatever angle you need (in the case of a miter sled).
Assuming I am working on a sheet or something where a mitre saw would not help...
Usually the factory-cut edges are straight and square, so you can put one factory-cut edge up against your table saw's fence, make the cut, then rotate the workpiece so the adjacent factory-cut edge is against the fence.
That said, the table saw is not my first choice for breaking down full-size (8'x4') sheet goods. Usually I'd rather lay the sheet down on a piece of foam on the ground and use a circular saw with a zero-clearance straightedge guide (or a track saw) to cut the sheet down to rough dimensions, then trim to final dimensions on the table saw if necessary. You can also use a parallel edge guide like the Kreg Rip-Cut along with your circular saw, using factory edge as your reference surface rather than using a straightedge or track saw that runs slightly longer than the length of your workpiece.