I don't use FWD unless I need it. If I have it in FWD while finish mowing it tends to scuff more. I do use FWD when finishing mowing on steep hills, since I have trouble getting traction with just the rear wheels in a couple of places and the brakes work much better going downhill in FWD, so it's safer. I shift in and out a lot when mowing my back yard because I need FWD on the hills, but I shift out when turning around to avoid scuffing the grass. Otherwise I use FWD when I'm doing anything on hilly ground and/or in the mud, when plowing, tilling, using the dirt scoop and grader blade, and of course whenever I start loosing traction doing whatever.
Since there are no lock out hubs, the front drivetrain is always turning anyway. As long as you're not on a hard surface, there's not much stress (if any), so if you're not worried about scuffing you can just leave it in FWD all the time. The main thing is to avoid having it in FWD for extended periods of time on a hard surface, especially if you have a front end loader full of something, since then the front tires won't slip very much.