Here is a little jig I made to check parallelism of the blade, slots, and fence after using measuring tools to do the actual setup. This is not to set the parallelism, just to check
I started with a piece of 3/4 inch hardwood about 10 inches long and ripped it to 1 1/4 inches. I then took a scrap of 1X8 about 16 inches long, set the blade to 30 degrees and using a tenon jig ripped both sides of the scrap to create a 60 degree angle on one side. I then cut a 45 degree on both edges leaving about 1 inch at the edge. I then use CA glue to connect the two pieces together trying to center the 1X8 on the 1X1 1/4. Lastly, I placed a flat head screw in the 1 inch area at the end of the board. I added some masking tape to the back of the 3/4 piece so it would fit very tightly in the miter slot. It should be tight enough that it doesn't slide easily.
Setting the blade to 90 degrees, I place the jig in the miter slot at the back of the blade and very gently scrape the blade against the wood making a small mark (flat top blades work best, make sure to unplug the saw). I then flip the jig over and move it to the front of the blade, rotate the blade so I am using the same tooth and scrape again. If the marks align perfectly then the blade is parallel to the miter slot.
Now setting the blade to 45 degrees, I place the jig in the miter slot (this time with the 60 degree edge of the board to the blade). Repeating the scraping as above, checks the 45 degree parallelism. The pointed edge of the board makes it very easy to see if the marks align
Finally, with the jig in the front of the miter slot, I move the fence very close to the head of the screw, tighten the fence, and using a feeler gauge measure the distance. Move the jig to the back of the miter slot and measure again. I prefer to have the back of the fence .001-.003 further away so I adjust accordingly.
After several uses, the multiple scrape marks obscure each other so I break the glue joint, cut off some of the 1X8 and re-glue.
For the life on me, I don't understand using "scrape" marks on the blade when the screw is used on the fence to check parallelism. Same conditions exist... whether it's the plane of the blade or the plane of the fence.
When the blade is at 90 degrees to the table, theoretically the trunnions do not come into play, since there is no tilting going on.
When the blade is tilted is when the trunnions are needed. The trunnions create a theoretical axis of rotation a few inches below the table surface depending on their radius. This axis must be parallel to the miter slots in top view and parallel to the table in side view.
The side view is what becomes critical when the blade is tilted. If the axis drops at the rear when the blade is tilted, the blade will bind or rub on a bevel cut. If one leg of the rear trunnion is higher or lower than the other it will affect the smooth operation of the tilting mechanism. The side view trunnion axis orientation won't affect the cut at 90 degrees, it just means the blade will be slightly higher or lower at the rear .... or front, no big deal.
Measuring this theoretical axis is the issue when the blade is tilted at 45 degrees, since there is no actual reference, only the plane of the blade. There are precision ground plates for setting up the trunnions for those who want more accuracy:
The Setup & Sanding Plate (item SSD-010) makes table saw, radial arm saw, and miter saw calibration faster and more accurate. Like our table saw blades, the disk is laser cut from premium cold-rolled steel plate, and is guaranteed to be within .002" flat. It will allow you to fine-tune your saw for perfectly effecient and clean cuts. It's ideal for checking a machine's arbor for square or setting your miter gauge to the precise angle.