I also use olivette type shot ( similar to the pole float ones ) or heavy metal putty to pin the pop up rig down about an inch or so from the hook. you do not always want the bait popped up the full length of your rig, ie on a clear bottom.
If you use shot, then I suggest that you do not attach it direct to your hooklink as it can give you a weak spot.
I have seen, but not tried yet, where you attach a small piece of braid with a water knot about an inch from you hook, then attach the shot to the braid about an inch from the waterknot. this is great when using fluorocarbon hook link on a pop up as you have not weakened the fluorocarbon by pinching on shot, and the suppleness of the braid gives a more natural movement to the popped up bait, which is often lost with the stiffness of the fluorocarbon line.