I have to absolutely agree with this. Precision ground stones are an important tool for any machine shop. I had a friend who used his surface grinder to go over a pair of inexpensive, hardware store stones that were far, far from flat. He ground both sides (coarse and fine) and the two edges. He left a shoulder on one edge so I could do corners.
There is no real substitute. They are great. Before using them, they are rubbed together to remove any stray grit, dust, or metal from the burrs they have previously removed with one of them. After rubbing them together for two or three strokes, they slide across each other almost like an air bearing. Then you use one of them on the table or any other flat surface to remove any raised areas. It works like a charm. Other places where I have used them include: my squares and protractor heads, 1-2-3 and 2-4-6 blocks, milling vise surfaces, my better rulers, my rotaty table, etc. One place where I do not use them is on my shop blocks. There are stones made for that. I do not know what the difference is, but I do not want to introduce any wear or scratches on them.
Previously I had a stone that I used on my mill's table that was fairly flat, but it was not ground. It did not do any noticible damage to my mill's table, but I kept it's use to a bare minimum. I do not have to do that with the precision ground stones. Even with moderately heavy pressure, they simply can not remove any of the table itself, just any high spots. I use them much more than I ever used that unground stone.
If you must use an ordinary stone that is not ground flat, DO find one that does not rock when placed on a flat surface. And one with a fairly fine grit.
This is a job for precision stones. Those norton stones you reference made flat in a surface grinder using a diamond wheel. These are very flat and will only remove burrs and crud. You will need a set of two. The surface of the stones will load up with chips as you use them. Sliding the second stone across the loaded one will drive the chips into the surface and out of harms way. As you rub the stones together they will slide across each other as is they were made of glass.
Sounds hokey but they are very effective and can be used on angle plates, parallels and just about anything. You will see where the burrs or high spots are on any surface with no damage to that surface.