When you are making a formula for a compound and you cross over the valences do you ALWAYS divide the numbers?

Or is that only with metals and nonmetals or somthing :sWhen you are making a formula for a compound and you cross over the valences do you ALWAYS divide the numbers?
In the example Fe is +3 and O is -2 so it is


Fe2O3


You just cross over the valences.





But with Mn +4 and O -2


the result must be divided by two


MnO2