I use one if I'm actually drawing in photshop and it's very detailed. Usually I just sketch it on paper and scan it in, because the tablet takes a long time for me to use.
I use primarily Illustrator with a dash of Photoshop... I just got a Wacom tablet about a month ago and am learning to use it, but I still do 99% of my stuff with a mouse (or I also scan it in)...
I do a mix of rough sketches on paper first or directly in Photoshop/illustrator with my bamboo fun tablet. Same goes for the linework and coloring, I can either do it all in PS or AI or mix between the two. Whatever I feel comfortable with on a particular design I guess. No methods, just madness.
Given, I haven't had a shirt published yet, but all of these are free.