Centring on GMT is conventional and probably makes more sense than anywhere else. Australians may beg to differ though:
http://img.memecdn.com/australian-world-map_o_1081710.jpg
(I quite like that map, actually.)
http://img.memecdn.com/australian-world-map_o_1081710.jpg
(I quite like that map, actually.)
But GMT is still Eurocentric and unimportant from the perspective of an individual looking at a map, I do wonder if that will ever change?