Welcome Tony!
I don't know of any rules, but other factors like page size and use play a role too. Perhaps you can enlighten us?

Hi again!

Thanks for your answer. What I had i mind was that when working with grids there is a recommendation to use for example a 12 or a 16 columns grid.

What's your paper size? When creating a 12 column grid on an A5, it would be a little overdone ;-) Is it a book or a magazine or...

Allright. It was just a general question about modular grids but I get your point.

It's always a good idea to divide the grid into equal widths. Having said that, if you want to create something out of the ordinary, an unequal grid might do the trick and have a fabulous effect.