I've only seen this once and it was on the GCN MC WR ghost.
You can change the direction you face before the start of the race. That way you can start your wheelie immediately after time starts rather than having to wait until you're facing the desired direction (perhaps a little more to the left or right).
This is the way I do it and it works great (very easy), though I'm not sure it is required to be this elaborate:
1. As soon as you have control over the character, tilt the joystick in the direction you want to rotate.
2. Move the joystick around the top of it's holder. For example, if you intend on facing more to the left, you would start with it all the way left, then left/up, then up, then right/up, then right. You don't pause at each direction, you just quickly rotate the joystick around the top.
3. DURING the movement described in step 2, attempt a wheelie. You character will yell at you and you should rotate a little bit.
I haven't experimented with it a whole lot, but you can change the degree to which you turn. I think that if you hold the direction longer before rotating the joystick, you will rotate more. I'm not entirely sure, however.
I hope this is useful information.