The only die that contacts the case is the full length sizer die, the seating die does not contact the case until the mouth of the case contacts the crimp portion of the of the die.
The exception to this would be a Factory Crimp Die (handgun variety) when lead bullets. In my experience I have found that due to the loose tolerances Lee uses in the manufacture of FCDs you can easily end up with a tight one that contacts the case body quite regularly.
Certainly one could load 1000s of rounds with a normal to loose FCD never touching a single case body (loading .38 Special or .357 Mag using your own cases for example). On the other hand, when using a tighter FCD to reload semiauto calibers with some brands of cases you might find that the post sizing ring touches every case. Range brass that has been bulged slightly can also cause this.
In my case, my .45ACP FCD is somewhere between .0005"-.001" on the tight side. When using Win, FC and PMC cases this FCD post sizes every single case. If you pull the bullets you see a tiny ledge where the case swaged the bullet. With RP cases, it sizes none (yet another example of RP cases being thinner than any other major brand). This little bit of post sizing is completely harmless in my instance and has no effect on accuracy (sub 2" five shot groups at 25 yards). My .380 FCD sizes 5%-10% of the cases that pass through it as well. 9mm, not quite as much (unless I get a batch of Glocked cases)
I am not trying to create a pro/con FCD debate, I was just making a small clarification. If you do not use an FCD- or bulgebuster-type die then of course F. Guffey is correct as usual, only the FL sizer die touches the case body.