In general, we carry two types of tees & tanks:
Solid colors which are usually 100% cotton unless otherwise indicated, and heather colors which are polyester/cotton blends.
This is is important to understanding what level of "shrinkage" to expect, and how to prevent it!
Personally with all my cotton tees even though I have selected the manufacturer (out of testing over a dozen) that shrinks the least, I don't put them in the dryer for at least the first several wearings. And it helps to wear the item for a few times before even washing it! After it stretches out and relaxes a bit, while forming to your body's curves, then you can wash it. Use a good fabric softener that will keep the fibers relaxed. But hang it to dry for at least 3-4 wash cycle before attempting to dry it on low to medium heat with your other stuff (let's face it, nobody wants high maintenance tops forever!).
But if it does shrink at all, just fling it around really hard after a wash cycle while wet and alternating holding each of 4 sides of the garment (this will stretch all the fibers), then hang dry and it should mostly return to its previous shape!
Polyester/Cotton Blends (Heather Colors)
These shirts are softer, fit better if you're a bigger guy, and shrink very little at all - even if you always run them through the dryer. These are personally my favorite - they have a better fit, and are very low maintenance. If only there were more guys in the world like this, eh?