Having repotted a wide variety of the more common genera, I can say that I've only trimmed roots on standard cyms. It would be impossible to repot a rootbound cym without breaking a lot of the roots, and those roots (which will rot anyway) are better off trimmed. What I did was cut off the lowest 1" of roots, and got rid of anything that wasn't still attached. Anyone who's had the unfortunate experience of repotting such a cym will know what I'm referring to.
However, I would suggest this rule of thumb--if a healthy root cannot be repotted without breakage, then that root is better off trimmed. I imagine that if you have a very large cattleya that is very very rootbound, it may be better to cut off some of the roots like above than to cram them into the pot. I would not trim roots on a regular basis.