Hello,
I just created this little plugin to help to quickly change the grid step.
Just click on the step value you want and the grid step is immediately changed ; the plugin window can stay opened, and you can also change the step while drawing a polyline (or while other objects drawing)
The item is in the
plugins menu.

Grid step are currently fixed, but maybe I'll add a way to edit the value in a further release.
step are 0.10 ; 0.25 ; 0.50 ; 1.00 ; 5.00 for mm (and other except inches)
0.005 ; 0.010 ; 0.025 ; 0.050 ; 0.125 for inches
Feel free to ask me if you want other steps value for inches ; I'm not familiar with this units, and I'm not sure that this settings are the best for inches users

enjoy
++
David