As Derek mentioned, the Google Maps scale are the de-facto standard for web mapping. But there definitely are some situations, in which you can't use those scales and projection.
Firstly, you should use those scales and coordinate system that matches with other online Web maps and services. In our case, we had to work with services which are in a certain projection that has been chosen as a National Standard. This is an LCC projection, and hence we have had to use a different coordinate system and scales.
Secondly, according to our national map policy, there are certain features that we cannot show beyond a certain scale. Hence we had to chose a scale level which met our statutory requirement.
So to conclude, the only standards that you should be concerned about, are those that you will have to interact with.