That's one of the most comprehensive water quality statements I've seen from a municipality, and the first one I've seen to quantify hardness. They say it's officially "soft to moderately hard", and later say "4 to 5 grains per gallon". One source says, however, that hardness is defined as:
- Less than 1.0 [grains per gallon] = Soft
- 1.0 - 3.5 = Slightly Hard
- 3.5 - 7.0 = Moderately Hard
- 7.0 - 10.5 = Hard
- Over 10.5 = Very Hard
so if your water is typically 4 to 5 gpg, it's hard enough to make many people want to soften it.
That aside, remember that corrosion in a water heater is normally a galvanic action, inhibited by the anode rod in the tank. Theoretically glass-lined tanks are immune to this, but that theory doesn't always translate into practice. If it's an electric water heater, be sure it's grounded properly. You can monitor the progress of potential internal corrosion by removing and inspecting the anode rod. It's not easy, especially if it's covered up by the tank's construction. If you see the anode rod disintegrating significantly, it can be replaced with new -- as long as IT's corroding, the theory goes, your tank is NOT. As cacher_chick points out, a good-quality electric tank, properly maintained, can last up to 20 years. My mother had one in her home that was over 30 years old when I left. You can also get an idea of what's going on inside the tank by periodically draining a couple of gallons and seeing what comes out.
There's a good site describing the care and feeding of water heaters at
http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/what-kills-water-heaters.html.