I'm late to the discussion, as usual....
It depends on what you need to measure and how accurate your measurements need to be.
A cast off piece of countertop is just that... a countertop. It's not a precision tool like a surface plate.
Surface plates are checked for overall flatness and repeatability. Overall flatness is simple, that is how much the plate is concave or convex. Grade B for a 18 x 24 (typical bench top size) has a flatness tolerance of .0003". Grade A is half that and AA half again.
Repeatability is slightly different. That means any spot on the plate can measure within a certain spec. Using the same size plate in Grade B, you would be able to measure at any spot within .00011". Grade A is .00006" and Grade AA is .000035".
BTW, plates do wear with use. Shops with a good calibration program have their plates checked on a regular basis. BTW, that's one of the things I do for a living. If someone really wants to know how it's done, I'll be glad to share.
Better plates have higher amounts of quartz visible in the granite. The pink Starrett plates are very hard, as is the Rock of Ages plates. Plates that are jet black are cheaper and a lot softer.
For home shop use, you can buy a cheap Grade B plate from someone like MSC. Keep it clean when you are using it and covered when you aren't and it will serve you well.