If you're stance width is too wide, it also makes a board harder to control. Stance width can be an inch or so wider than your shoulders, or if you measure from the middle of your knee cap to the floor that's also a good metric for stance width. It really all boils down to comfortability though.
As hobo mentioned, your bindings should be centered on your board with equal heel and toe overhang. What I do is I adjust the heel cup so my boot is centered on my binding, then I put the binding on the board and center that as well. Lastly I throw the boot in and ensure that there is equal heel and toe overhand with everything centered.
I have a pair of flow NXT-FSE bindings and no matter how I situated the base plate, I either had too much toe or heel overhang. The remedy here was to move the heel cup forward, center the binding, adjust the toe and heel sliders to match the width of the board, throw in a couple of bolts so I could gauge my boot overhang, then once it was perfect, mount the bindings completely. It's a similar process for all bindings.