There are flow sensors in tankless units so if you are simply shutting down the valve on the water supply and then reopening it I wouldn't be too concerned. That being said shutting down the water supply can change the pressure in the system as temperatures of the water change. I don't think it would cause any trouble. But, anytime you close the valve the water is in a closed system. It's a good idea to install a potable water expansion tank.
Isto é um potável water expansion tank
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They are almost exclusively 3/4" NPT. You would need to measure your static pressure(pressure with the taps closed and the main open) with a gauge. Then pump air into the air nipple (bottom of the tank pictured above) with a bicycle pump or preferably air compressor to match the static pressure before installing it. It's a good idea to install a service valve as well.