A filter of some sort wouldn't be a bad idea for the front either, Nissan just didn't bother. You want air to freely pass through there, but you don't want anything else to pass freely.
But far more important than the filter is extending it up above where you might expect any water to reach. Don't put a filter on the axle vent, put a tube on it and run it up to a higher level.
My diff breather is run in behind the tail light, and I put a small fuel filter on there. In addition to keeping bugs out of there it should also block water from getting in should water reach it, not that there is much risk of that.
Pretty low risk of anything getting in that shouldn't, be it a bug or water, but little filters for those hoses are cheap insurance that they stay clear and allow venting.