I find that with many t-shirts, an XL becomes an L after a washing. Therefore I tend to get XXL, unless i can hand check the shirt for quality. high quality XL's typically wear OK post-wash, so if it's thin and 100% cotton, I go UP a size.

For instance, got a XXL from OpenBSD, it's a bit big after washing (hottest setting). I got an XL SuSE shirt, it's a bit tight in the shoulders after washing. Bought both over the net, if I coulda felt em, both would fit a bit better I suspect (XL OBSD and XXL SuSE).

Somewhere between XL and XXL is perfect, depending... ;-)