Thad; a crimp should be enough so that all traces of the belling of the case mouth by the expander die is removed.
The bullet should be held securely in place by the crimp. The common test for this is to push the loaded cartridge, bullet first, against the loading bench. If the bullet slides deeper, the crimp is insufficient.
An important corrolary of this is that, if shooting a 'small pot' cartridge, such as the 9 x 19, which is VERY sensitive to bullet set back, check the OAL after cycling the loaded round through the action several times. It should not be shortened. Further, at the range, keep the top round in the magazine there for 3 or 4 firings (by ejecting it from the chamber and reloading it in the mag, and putting another cartridge on top to feed into the chamber). This process should not cause the cartridge to shorten either.
Some calibers require a heavy crimp; the .357 Mag and .44 Mag are good examples. A heavy roll crimp improves the precision of bullet pull.
Taper crimp auto pistol cartridges. This is not so much because of the polite fiction that the cartridge headspaces on the case mouth, and a roll crimp will defeat this, but to make the cartridge smooth so it will feed more reliably.
Don't believe me about the case mouth headspacing being a fiction? Well, it is the case. The .45 ACP actually headspaces on the extractor, as that is what is holding the case against the breech face. True, if you drop a .45 ACP round into a bbl. the case mouth will touch the ledge in the chamber.
You see, they have to build in a clearance between the case mouth, as it is really positioned during the firing cycle, and the ledge in the chamber. If they didn't, the slightest bit of junk which got deposited on the ledge would hold the case back, and not let the slide go into battery. Honest.
If you ever reload rifle cartridge cases, I highly recommend the Lee Collet crimper die. This crimps the case from the side, rather from the front, and yields a much more uniform crimp, not at all dependent on case length. I had a lot of trouble with brand new .223 brass varying in length. I could have trimmed all of them, but decided instead to try the Lee Collet die. It worked very very well. Hope this helps, Walt