The book Architectural Design and Drafting, 5th edition (page 885) suggests that numbers be used to define grids top to bottom and consecutive letters be used to define the grids left to right. What's funny is the example shown on the preceeding page has it just the opposite. The letters define the top to bottom grid with "A" at the very top then running consecutively towards the bottom of the sheet. Go figure.
I guess then it can be either. It must depend on individual office standards?