I don't know when 'official" D&D went from hexes to grids, I wasn't really a part of the transition. Grids have always been a part of the game I suppose, maybe because of the graph paper mapping thing, but when D&D evolved into more of a tactical combat game than a game of exploration and abstract combat one would think hexes would be the way to go.
Apparently some folks in charge didn't think that.
I do know that Avalon Hill's early war game Tactics II (1958) used a grid map. By the early 1960's they began using Hexes for the basis of their war game maps. Their game Outdoor Survival used a hex map and that map later was used by Dave Arneson for his early D&D RPG games. From there the hex map worked it's way into the RPG hobby ie. Judes Guild and various wilderness maps from TSR, Traveler, and pretty much every war-game etc . (though the B2 adventure Keep on the Borderlands had it's wilderness laid out in a grid).
Now when I was younger and saw the D&D game for the first time and saw sheets and maps based on hexes, it added to the mysterious allure of the game. Again, D&D (Holmes basic) was nothing I've ever seen before. Along with 'role-playing' and funny dice, there were minis and hex maps. Talk about opening doors to your imagination!
So why did a game, which evolved into more of a tactical combat game devolve back to a grid format?
Everything I've seen come out from WOTC 4th ed (even 3.5) is printed with a grid. Sounds backwards to me. If the 'constant distance of a hex map is desirable for games in which the measurement of movement is a factor' why use a square grid? By comparison, a square grid give you different distances from the center points of adjacent and diagonal squares. If I recall, somewhere in some version of D&D special rules were created for diagonal movement within a grid. Why write an extra rule and complicate an already complicated game system when you really don't have to. WOTC (and other games) just print their square 'grid' over their battle maps so why not just print hexes? Do players even still map those gaming sessions or do they just throw down a battle map?
I'll never understand that decision path so if somebody knows, please share it with me.
When I began making my gaming table, I had planned on going with a grid. But the more I thought of it, the more I felt that going with a grid was just counter-intuitive. So I went with a hex board...and I'm glad I did. I'll always prefer hexes of grids.
So hexes or grids? Which is your preference?