Again, while some projects may warrant simulating poor optics, this is not generally desirable. On the other hand, even the best lenses will display a degree of CA in very high-contrast areas at certain apertures, which is often only visible when viewing photos at 100%. A tiny bit of CA can go a long way towards making a render look like a photo. My guide here, though, is that it should be barely visible to the naked eye, and only at 100%. Ideally, viewers should be able to perceive it without quite being able to put their finger on it.