I wouldn't recommend using a thickness planer for making tapers. It will more than likely catch the thin end and destroy the lam. The newer ones that have a spiral cutter head may not be as bad as the ones with straight blades, but I still wouldn't take the chance.
Tapers can be done on a jointer, if you don't have a thickness sander. Mark the center of the lam, then make a mark at 1/3 and 2/3 of the distance from the end to the center. Do the same on the other side of the center mark.
If your lam is 66" long, then you will have a mark every 11". Then you plane from the end to the first mark, turn the lam around and plane the other end to the first mark. Now start again from each end and plane to the second mark. Lastly, plane the whole lam from end to end and you will have a double taper.
The safest way to do this is to start with a thick piece of wood. Once the tapers are cut, you can cut the tapered side off at the thickness that you want using the tablesaw, or better yet, the bandsaw.
One thing to remember. Be very careful when doing this kind of planing. Use a pair of push pads to control the wood and keep your fingers away from the cutters!
Dave.