Aligning yourself in the middle of a straight taxiway isn't a problem. On the 747, if you put yourself in the middle of the taxiway as you see it beyond the nose, you'll be in good shape. If you're the captain, you can hold yourself a little left of the perceived center, but not much, because you're looking at from an eye level over 30 feet up as I remember. If the taxiway has those little metal reflectors sticking up on the centerline, you can play games trying to keep one of the nose wheels hitting them. You can just barely feel going across them. Or, since the nose gear is two wheels, you can play with getting the reflectors between the two wheels if you're really bored.
Turns on a taxiway present a bit more of a challenge insofar as keeping the center line because the nose gear is behind you, but one develops a feel for it. A 90 degree turn from a narrow taxiway onto another narrow taxiway is a little challenging, especially if the taxiway you're on T's into the one you're turning onto. All you'll see out the front window as you start the turn is grass. Also, on narrow taxiways, you can't see the taxiway out the side window from your normal sitting position.
If you need to do a 180 on a runway, you need to remember that when you first get yourself on, say, the right side of the runway, the nose gear isn't there yet, and the wing and body gear have even further to go. You have to run yourself along the side of the runway a sufficient distance so that the airplane is aligned with the edge of the runway before you start your turn.
Personally, I found taxiing a 747 one of the more fun things in life.