Well, Robert E. Howard's map is "left justified", but it's pre-cataclysmic Europe + bits of Asia and Europe. Lloyd Alexander's Prydain is "left justified", but it's a fictionalized Wales. I'm just not convinced.
Am I guilty? I'll check the maps I have convenient.
Full continent 4
No ocean or minor intrusions of seas 3
Oceans on three sides, north vanishes into ice cap 1
Oceans on three sides, east ends in a land edge 1
Ocean on left 2
Ocean on right 2*
* - includes one where right is south
Ocean on top 1
Ocean on bottom 1
Lake on bottom 1
Ocean on lower left 2
Ocean on upper right 1
Looks like I don't have a strong bias, except that today I really want full bodies of land, be they islands or continents.
I'm not really sure that there's a dominance of "left justified" maps after you throw out those that are fictionalized versions of real world places. Google image search for 'fantasy map' turned up only three on the first page of results, all of which were Middle Earth.