I've always just used Default Shepard as he appears on the cover of both games, Mainly because all of the custom faces look horrible, but partly because I think the Default Shepard is very well done, sexy and fits all the dialog he delivers perfectly. I have also developed a major man-crush on him over the last two games, so there is no way I'm changing him in ME3.
OT: I don't think they need to have a reason for it in the game itself. It would be just fine if you got to re-customize him before the game and but the game still acts like you always looked like that.
OT: I don't think they need to have a reason for it in the game itself. It would be just fine if you got to re-customize him before the game and but the game still acts like you always looked like that.