How much would a flat square be curved, on Earth, if is was 512x512 miles? Would you even be able to notice? Besides, this is a WW2 game, and we don't have weapons that can reach out far enough for it to make any difference. All the speed calculations are done for you, and as pointed out, they are pretty dang accurate. Would having any more precision by knowing the precise terrain curvature give you opportunities in the game that you don't already have?
A degree of latitude is 60 nautical miles (69 miles, 112 km), so if you were standing at the center of the grid, the closest edge would be curving away (down) by about 3 1/2 degrees, and the farthest corner by about 5 degrees.
Mike
Edit - also, the Earth isn't a perfect sphere - it bulges at the equator due to centrifugal force (the spin) and a very small bit because of the moon. As a result, the curvature would not be identical in all directions. As a tidbit found while looking up some of this, although Mount Everest's peak is the highest point above sea level, Mount Chimborazo in Equador actually reaches farther from the center of the Earth due to the bulge at the Equator.