How about a checkbox on the monitor window labelled "always monitor this grid" so that no matter what grid you are editing, if this box is ticked, pressing the monitor icon or CTRL-M will always bring up the monitor window for the grid you were monitoring when you ticked it. This is great for when you are making changes on one grid where you want to quickly preview the result but the grid you are editing isn't the one you need to see - otherwise you need to make your change, switch grids, monitor, switch grid back, make more changes etc.
Ben