i'll assume that you just want smooth movement....
there are two easy ways to ensure that you get whole pixel movement (to avoid the 'wavy' anti-alias text issue).
one way is to add this expression to the position property:
offset=2; //value in pixels per frame
set the credit layer's position at it's inpoint to off-screen bottom (or where ever you want it to start), and it will march up the screen until the end of the comp/layer. adjust the offset variable as needed, but you'll probably need to keep it less than 5 or 6, or the text will get start getting hard to read...
another way that is easier for many to work with for timing is to create 2 keyframes for position that are 1 frame apart. set the y value for the second key frame to be the number of pixels per frame movement that you need (say 2 pixels).
then add this expression:
the movement will not happen until time hits the first keyframe, so timing can be adjusted in a more familiar way, then the code just continues that movement. you might add this animation and expression to a null, then parent the credits to the null to make it easy to adjust the credit position separate from the animation.