I would only worry about it if it is going over 210. The factory t-stat will climb a little and then drop back down when the stat opens. This is normal.
And check your fluid level, and if it's low, or you are boiling over, replace your radiator cap.
If it's getting over 210, I say replace the thermostat immediately. I recommend the Cummins 'stat. It will fluctuate, but flows better than the aftermarket ones and reportedly fails open-mine did (not closed overheating your motor) I have a Napa stat in mine, but it's gotta go.
The stat is super easy, and a very common cause of engine failure. If that doesn't do it, consider a coolant flush and clean the gunk out of the radiator fins if there is any. This will cover most of your possible problems with very little money spent.
If it still is overheating after that, maybe you have a bad water pump impeller, a restriction in your cooling system, or perhaps an engine problem like a blown headgasket. These scenarios are much less likely, though. Very likely a t-stat will fix you right up.
Good luck with it.
PS use distilled water. Acidic water can cause excessive corrosion (I've seen it eat through a head to open air) and hard water can plug thing up badly over time. Distilled water will absorb excess minerals keeping your system running clean-and your heater core flowing.
