Another thing to keep in mind: Even if you have the most accurate scope in the world, have you stopped to consider the accuracy impact of your probes, connectors, coax, etc?
If you want to measure to a fraction of a percent, a small amount of impedance mismatch here, a bit of parasitic inductance and capacitance there, and boom, you lost your fraction of a percent. Think about the fact that you have to manually go in and tweak that 10x probe. How on earth are you ever supposed to get accurate measurements out of that?
This is a similar reason why things like spectrum analyzers can't measure to extreme accuracy.