many people shut off their main water valves when they are away for a long time. You need to decide for yourself if you want to prepare for potential failures. I shut off mine whenever I go vacations.
Your water heater does not lose water whether it is on or off. It is a close system. Did you ever see steam coming out of your water heater or of your faucets? Your water heater won't lose a drop of water even at normal working temperature.
You may feel a little bit of anxiety when you do it first time. After that, it is easy to depressurize your waterlines when you are away for a long time.
Best regards,
Alex
