>> probably missing the obvious.
>
>> For example if I buy a 2'x 4' sheet of wood and there is not a perfect >90
>> angle on at least one angle, will not all my cuts that are up against my
>> fence on the table saw be off?
>
>For "practical" purposes the 'factory' edges of sheet goods are considered
>"square" ... and are generally "square enough" for woodworking purposes.
Unless one is buying seconds which have not been cut square; remember he said 2'x4' not 4'x8'.
Often, for utility projects, I buy from Shurway which sells seconds and trims. They have cuts in 2'x4' pieces which can easily fit in my Prius with the advantage I don't have solicit a pickup to haul it or to struggle with a large sheet of 3/4" plywood! If I use these for anything that matters, my wonderful (old-time and good quality) carpenter's square helps to ensure the wood is square.
Glenna