CGR's post is most informative, but...

Only SOME brands of servos have the servo arms that will angle slightly when rotated to the next arm (if you use JR, you're out of luck)

And you don't ALWAYS want the arm to be at 90 degrees.

When the arm starts at 90 degrees from the pushrod, you will get the most amount of travel, but sometimes, you're stuck between a rock and a hard place, IE, when one hole on the servo doesn't give enough throw and the next one out gives you too much.

A blank arm like AA5BY suggested is a possibility, but it is easier to use the arm you have and rotate it off the 90 degree mark - which will give you less throw.