Increase/decrease decimal should work regardless of the active cell
If you have a mix of text and numbers selected, then increase/decrease decimal (in the Home tab) will not work if the active cell is text, but it will work and be applied to all numbers if the active cell is a number.
This issue comes up when you have a column of all numbers except for the title at the top. Then when you select the whole column and try to increase/decrease decimal, it will fail until you activate a numerical cell.
Kenneth Barber commented
For those that don't know what an active cell is, it is the white cell in your selection. You can change the active cell with (Shift +) Tab and (Shift +) Enter.