As
JohnnyFive said, experiment a bit and see what works best for what you are doing.
That said, there is a whole lot of info out there about how to print, and an awful lot of it is put forth like it is written in stone, without ever bothering to mention that it is talking about printing Plastisol. That is because Plastisol is all there ever used to be, and is still the most widely used ink. Tell tale signs are talk about wanting a thick coat of emulsion on the screen (thick gasket), high screen tension, stirring the hell out of the ink, and off contact.
The general wisdom is that water based should be printed On Contact so that the ink penetrates into the fabric (with Plastisol one deposits the ink on the surface).
Zero off contact means that screen tension is less important, because you are not pushing the screen down to the fabric and requiring it to snap back as you slide the squeegee across it. Some people use a small off contact with water based, like no more than 1/16 inch.
I've found zero off contact to work best for me. YMMV
One time when a bit of off contact might give better results is when the print/ink requires you to Print / Flash / Print in order to get opaque coverage. But for "normal" dark ink on light fabric, direct contact is best.