When attempting to use repeat grid to create column guides for responsive themes, I found that it is not possible to snap to fractional x or y values. This means that if a column is a fraction, you have two problems.

First, when creating your repeat grid, you create the first column of (say) 67.5 px width. Then you create your second column of 30px and try to snap it up to the right side of the first column. It unfortunately will be .5 px to the right of the boundary instead.
OK, fine, to remedy, I simply shave off .5 from the X value of the second column.

Then, I select both items and then repeat grid. I create a few of these pairs of columns and then go to remove the gap that is created. The gap will SAY it's 0 px, but it's actually .5px. It won't snap to the actual boundary of every other pair of repeated columns.

So while the main problem is I can't use repeat grid to quickly make something if there are fractions involved, the root problem is that the tool does not handle snapping to fractional values! I guess this would be correctable if you let us define the pixel grid size as you can in Illustrator.. Since this is a vector tool and we're dealing with responsive web design where percentages need to be reflected, we really need finer resolution for snapping.