I found the Carhartt shirts and jeans sizing to be pretty accurate. I am at the 3X end, and the shirts I purchased at the start of summer have not shrunk at all. By comparison, the Beefy T shirts I used to buy would slowly shrink over time. We also do cold wash and low heat to dry, so I don't think it was our washing habits.
I started by purchasing one or two shirts with free or cheap shipping, to verify size, and then follow with a larger order. I also have the luxury of passing down anything to my son, who is one or two sizes smaller than me.
We order quite a bit from Sierra Trading Post, and have been pleased with their characterizations of sizing and, at times, instructions to order one size larger (Mostly in shoes and ladies' wear).
I was also a bit concerned about the irregulars, but they have held up just as well as the first quality. I tend to get stuff on the front of my shirts, so using first quality to get stained, etc, was not a better solution for me.