FBinNY 

I build with a large variety of rims, focusing on relatively light tubular rims.

Overall the first thing I consider is the quality of the extrusion, consistency of curvature, and accuracy of the joint. Quality issues that you won't usually see on heavier machined side rims, but may on light rims are twist, which results in a built wheel where one side or the other will rise or fall while the overall rim is true. This cannot be corrected except by extreme methods, and so can have me avoiding certain brands altogether.

Overall roundness or flatness are less important as they can easily be trued out and corrected within the tension tolerance, but this varies with the rigidity of the rim. It's easy to correct on a 310 gram tubular built 32h, but much harder with a heavy wired-on rim, especially if built with low spoke count. These must be close to round before building.

While overall roundness isn't critical local distortion, usually at the joint is. Often non-welded rims will have joint issues, and like twist, these cannot be 100% resolved by the builder. Years ago, when most rims were not welded, the general attitude among builders was that that the alignment was as good as possible when the joint was the worst area.

Unfortunately, IME it's difficult, if not impossible to really know rim quality until you build. Even with brands having good reputations, you can still find a problem rim, but if problems are more than isolated experiences, I'll abandon the brand, or at least he model.