If you have mineral laden water, it might be a good idea to flush the water heater a little before you brew. 5 gallons might be enough. If your water is low in hardness and alkalinity, you are good to go.
My last water heater lasted around 8 years. The plumber said to flush once a quarter to extend the life. Draw off 5 gallons into a pail and let it set for a day or 2. If not much has settled out, no problem. If it look slike mine, you will know what I am talking about!
A local brewery uses a hot liqour tank, that is at strike water temps. They don't have problems, but they have water from Lake Huron. I have gound water, which is full of minerals.