I haven't heard of any codes or regulations. I usually put it high because 99.999% of the time it will be used for the shower so it's better to install the valve in a place that's more convenient for shower users but not so high that tub users have to stand up to shut it off.
There is no standard. For a great recommendation, go to one of the better German faucet company websites and seek assistance. A few examples are Grohe, Hansgrohe, Dornbracht. Excellent customer service and sensational products.