QFT! My group uses Roll20, but the grid is just a guideline. Anything broken down to less than 5' is at the discretion of the DM (who hopefully apples a generous helping of common sense). Some of our areas may be slightly different than listed in the book (since Roll20 measures from the center of each square for point of origin), but it works out for us.Are you sure the group is actually using the square grid rules, or does there just happen to be a battlemat underneath the play area. There can be differing levels of how 'hard grid' a DM runs things. Are you marking intersections of origin for area effects and light radii? Because for groups I play in, the square mat is just there to help measure distance, not to cage folks into individual squares.
I suggest the OP talk to the DM and figure out how they run the grid rules. If it's fast and loose, don't worry about it, and think less of a grid and more of common sense. If it's strict, then learn the exact nature they are used (3E has the best grid mechanics that translate to 5E, IME, so many DMs seem to use those).