If your just towing on paved roads there's no need for 4wd. If you intend to get on steep gravel or dirt roads it may come in handy occasionally. One thing that often gets trucks stuck, particularly if they aren't towing and have nothing in the bed, is wet grass. As dumb as it sounds I once go a 2wd truck stuck in my yard on a slight incline with wet grass and had to pull it out with a 4wd.
Around here it snows and there are mountains. I'd say 90% or more of trucks here are 4wd. You can't find a 2wd on a dealers lot and you can't hardly sell one if you try; basically nobody buys them. So, resale value might be a bigger factor than anything.
As for a MPG penalty, there probably is one because of the extra weight but it is likely so insignificant you couldn't measure it.
