When I do portraits, that are critical, I rather use my lowly 50f1.8 at f8 (or the macro 100mm) or so with my 5d2 than the 24-105 .I almost always shift the focus point manually to where I want to focus, usually the eye. I just don't find the zoom to be as sharp off centre than the 50/100. But lets be honest...a school portrait viewed at 100% and the eye is not absolutely perfect in focus... well....when that happens I look at 50% which is much bigger than most prints I will make and then everything almost always look fine. I would say my success rate is above 90% and those that are totally off, quite rare....the other 9% is close enough not to be noticeable by any client in any case...but of course I notice it!
For stationary studio stuff I almost always use live view at 10x and then its much easier and I am more confident of 'best' sharpness.
I have not had my cameras calibrated, just lucky I guess, but the minute I thought something was off I would send the whole lot in to be matched, checked and calibrated...
I suggest you read the canon rumours article on lens softness etc to put things in perspective.....
And I am sure, that if you are like me, we are probably way too critical of our own work in any case....