I'm trying to get started with some wheelbuilding. I've got a Park Tool TS-2.2 truing stand. Can I just rely on the calipers to provide a guide for centering the rim and dishing rear wheels or will I need separate dishing tool?
A properly aligned TS-2 truing stand will show dish, as well as alignment. You rarely need additional adjustment after using it. However, if the stand is out of alignment, you may need to re-dish the wheel.
The easiest and most accurate check for TS-2 alignment is a properly dished wheel.
The best way to ensure that the wheel is properly dished, and thus an accurate alignment check, is with a dishing tool.
Bottom line, if you can afford the TS-2, a dishing tool shouldn't be a major expense, and is worth the cost, even if you only use it as a final, double check of the truing stand.
It wouldn't hurt to buy an inexpensive dishing gauge for double-checking your builds. I have an old Performance one that would likely cost about $15 today, and it's perfectly adequate for this duty.
However, you can get the same double-check by simply flipping the wheel around in the truing stand.
As long as you verify the centering you don't need to do separate dishing.
You will need the centering gauge: http://www.parktool.com/product/centering-gauge-1554-1
Do read the instructions and get comfortable making the centering adjustment. It's not difficult. Biggest thing to pay attention to is to SLOWLY lower the gauge into place.
Happy Riding! (and Wheel Building!)