It's a personal choice. Residential streets, in my experience, are dangerous because cars are pulling out of driveways and "getting ready" for the commute ahead (fiddle with CD player, light cigarette, put coffee in cup holder, scream at kids in the back seat etc.) - major lack of attention, easy to plow into a bike. You'll see arguments for and against helmets, but my argument here is: in some ways it's safer on a mountain bike trail than it is on a street with moving cars, so do whatever you would do in that situation.