How do pilots hold the runway centerline during takeoff?
Once you've lined up nicely on the runway, you only need the rudder pedals as they do provide enough nose gear steering for the takeoff roll even at low speed.


  Rudder pedal steering is available during takeoff, landing, and taxiing when small directional changes are required. Full deflection of the rudder pedals produces about 7 degrees of nose wheel steering (airliners.net).


The 7 degrees figure quoted above will vary by plane.

Tiller is used when tight turns are needed. Remember most airliners don't have a tiller on the first officer's side, yet they do takeoff without the captain steering for them.


(YouTube)

In the linked video we see the captain let go of the tiller as soon as he is lined up on the runway. Also notice the tiller move on its own afterward, which indicates that the rudder pedal [nose wheel] steering is being used.