Yup, you're right. One of the reasons to lap rings is to make sure they have the same center axis, and another is to make sure that the two halves mate properly.
There's a lot of things that can twist the tube body of a scope: receivers are tapped with a tolerance; bases are milled within a tolerance; rings are manufactured within a tolerance; and two piece scope bodies will have a tolerance which will be worse than a milled body, but the one piece body also has a tolerance in its milling. The worst part is that all errors are accumlative.
(Scopes as a system also have a tolerance that they are built to; for example, a hunting scope will generally have an inherent error of 1 to 1.5 MOA compared to a varminting scope which is controlled down to 1/2 MOA. This refers to the fact that the internals of a scope don't quite settle in the exact same place as before the shot. The more control over this error, the more you pay for the scope.)
The good news is that with modern milling techniques (CNC machines are almost everywhere now adays) errors are very small even when they are all added up.
For us normal guys, I would say you don't have to lap rings unless you reallly need to correct a problem, or if you have a rifle that experiences around 1200g or more for a recoil, and you didn't buy the high end Badger, DD Ross, Talley, etc. rings and mounts.
So if you put the rings together and they aren't round inside, then lap them. Otherwise, I'm sure they will be fine for you.