You want the the throws to move more up than down. This is to counteract adverse yaw caused by the difference in drag between the two ailerons in a turn.

Any slop in your control surfaces can be very bad! It can lead to flutter which can cause damage in the air and loss of control which often ends in a big crunch sound of balsa meeting mother earth. First thing, identify the play you mentioned and get rid of it before flying.

Some general rules to follow:
1. With your radio and receiver on, center the trim adjustment of the aileron throw.
2. Unscrew the conrol arm from the servo, remove it and re-attach it so it is centered with each arm as close to 90 degrees to the side of the servo as possible. Now turn off the receiver then the radio. This should leave the servo in the centered position without haveing to leave your radio on.
3. Line up each aileron with the center of the wing. Tape them in position with masking tape if necessary.
4. Check the mounting locations of the control horns on the ailerons. They should both be the same distance from the hinge line.
5. Make both linkages as close to the same length as possible - this will help make sure both ailerons move the same amount.
6. Re-read the instruction manual that came with the plane and make sure any specifics haven't been missed.