This was suggested in a different form before, but mostly overlooked, so I am rephrasing the suggestion a bit to make it more clear.
Placing blocks accurately in Mine-Imator is hard.
(Grid demonstrated by the first three blocks, because I'm lazy)
This example shows how I thought of it. (Putting lines around transparent blocks like tall grass would be overkill in my opinon.)
Two options, one for enabling the grid, and one for blocks to snap to it. The last one would greatly improve how blocks are placed, because at the moment, you need to fiddle quite a bit to get blocks where you want them.
An optional thing would be to change the color/transparency of the grid.