I live in New York, so assume WAAS should add to the accuracy. Is there a reason to disengage WAAS ever?
Theoretically, there's one set of circumstances where having WAAS turned on might degrade accuracy. If you have a 12-channel unit, and there are 12 or more GPS satellites visible, having WAAS enabled would "crowd out" one of the available slots for GPS satellites. If the missing satellite was in a key area for the best geometry, you could in theory get slightly more accurate positions by turning WAAS off to free up that one satellite spot.
In practice, though, this is so incredibly unlikely to happen that for accuracy, you're always better off having WAAS turned on. It will also reduce battery life slightly, but not by very much.