Getting body work down the side of a whole car exceptionally straight takes more than using the right sanding blocks, and more than just checking how the body work feels. The goal is to be able to look down the side of the car, and not see any ripples in any panels, not be able to see where panels start and stop, and completely flat paint.
There is one way to make sure larger areas are completely ‘straight’, and it also makes the work faster. That is to use aluminum flat stock or c-channel to check your body work. Spanning these across the panel will show you the high and low areas. Use these often while block sanding. It’s a waste of time to be blocking an area that is already low.
The flat stock should be able to follow the general shape of the panel with just a slight amount of tension. The c-channel can be used on much flatter vehicles. These can be bought at most hardware stores, like Home Depot or Lowe’s.