I had to add a switched receptacle off a 20A circuit yesterday to power some new workshop lights, and was really cussing the 12GA wire in each of the three boxes I worked in (old take off receptacle box, new switch box, and new receptacle box). Reminded me of this thread and how we were debating this a while back. There is a definite little penalty with 12GA in tight boxes and it became noticeable when working with so many boxes.
Side note/question is what rating you use for switches and receptacles. Contrary to intuition, 15 amp switches and receptacles are permitted on 20A circuits as long as there is more than a single receptacle. If a single you must have a 20A rating and the special 20A plug shape (since it's dedicated) but for multiples they can be 15A. Typical 15A receptacles are rated for 20A pass through. Be curious to know what other folks do when they run 12GA 20A circuits all over their house.