Some checker pushrods have markings on them that tell you the correct length.

Using checker pushrods with the actual valve springs in place is a sure fire way to bend the checker rods (they are not designed for that much tension). Usually the hydraulic lifter plunger moving down is not a problem as this is usually not more than 50 thousands and movement in the plunger occurs during normal operation.

The checker springs have very little tension (can be installed by hand) and allows the plunger to move up and down while cranking by hand, they are designed to simulate the oil pressure applied movement of the lifter(as well as checking valve clearance and other things), thus giving you a more accurate picture of the valve train geometry as it would be in normal operation.