If I add for example Total Sales (Sum([Sales]) into the Master Items library it would also be good to set the default number format, e.g. Money (no decimals!!)
I agree with both Fred and Simon. I don't see how this would be any different from colors for example.
It is a property of a measure which should be the same >95% of the time. The standard format for a visualisation is automatic, which inherits the format of the master item, but it would be possible to change it for a specific visualisation by changing the format to something specific.