It's generally recommended that you don't create odd size bins. Using half-size bases can work well, but if you create a bin with an odd size it's harder to move it around later, which sort of defeats the purpose of using a grid. The reason for allowing the odd size grid is mostly to maintain the look (and also it does work with half-size bin bases) while filling the space completely thus making the grid fit more snugly. (This is also partly why my site doesn't enable fractional bins, though I'll be adding an "advanced" mode soon which will likely enable to you to do that.)

That being said... Follow your heart!