usually it depends on what you are doing in the layout. It's easy to change, so you can change it at any time.
Usually multiples of 8 are what I end up using, mainly 16 or 32. But not for any special reason other than the usual sprite widths.
And if you haven't already noticed, there is a glitch in the grid boxes, so when you change one, you have to hit enter or click the other grid box, or it will reset to the previous number when you click anywhere else.