I don't have a 300 WM, I have a 300 H&H and brass is around $2.00 a case. So feeding it is not cheap.
What I do, on first firing, is put a light coat of lubricant on the outside of the case, and fire it. I also happen to be greasing the bullet, to reduce jacket fouling.
The base to shoulder distance on a belted magnum and belted magnum chamber is not controlled. Headspace is off the belt. Because of this dimensional mismatch, case sidewalls will be often stretched badly, unless by serendipity, all the dimensions just happen to be within tolerance. On a dry case with dry chamber walls, the case mouth/shoulders stick to the chamber, and when pressure builds up, the case sidewalls stretch, allowing the cartridge head to stop at the bolt face. That stretch ruins cases, many belted magnum shooters report short case lives because the base to shoulder distance for belted magnums can be anything. If however, you lubricate the case, on ignition, the case mouth will expand, the shoulders will expand, but they won't cling to the chamber walls as pressures build. Instead the case will slide to the bolt face and the shoulders will fill out, the case walls will expand, and there will not be any sidewall stretch. The case will be perfectly fireformed to the chamber. This is a practice Bench Rest National Champions do, to get a perfect, stress free case.
Greased cases and bullets also shoot fine.
Now, after you have spent all this effort to create stress free, perfectly fireformed cases, to set up your sizing die, go buy the Wilson gage for belted magnums, as described in this article. Also, read the article, it has very good advice.
Extending Cartridge Case Life
http://www.realguns.com/Commentary/comar46.htm
Because the base to shoulder distance is not controlled inside the sizing die, just as it is not controlled in the rifle chamber, you have to have this special tool to set up your sizing die. You do not want to push the shoulders back a quarter inch on those perfectly fire formed cases in some stupid sizing die. I would recommend pushing the shoulders back about 0.003".
This ought to make your cases last forever, as long as you don't push pressures and expand the primer pockets. People who buy magnum cartridges tend to push the pressures, because, they believe they would be "robbed" if they did not:
Or that they don't have to worry that much about marksmanship, because the cartridge will do the rest.