Steelrat--My first reaction to yr post was, "Gee, that seems odd." Consulted my own copy of Nolser 5th ed, and there it was on pg. 41, "We hasten to reccommend, however, that in cartridges over .30 caliber and all ammunition for big game hunting that you full length resize."
For big game hunting the reason would be for ease of chambering. When cases are neck-sized, even when used in the same firearm, they can sometimes chamber only with difficulty. If you need a fast second shot that might cause a problem. That's standard advice which you will probably find in a dozen other sources, as well. FL sizing loses you a little bit of accuracy, but not enough to matter if you have a game animal at reasonable range, that your first shot didn't quite dispatch.
Now, your .300 WinMag is not over .30 caliber, so Nosler's FL sizing reccommendation does not apply. However, that is a hefty cartridge, with a hefty powder charge, so you might need to FL size the cases anyhow, to make 'em chamber back into yr rifle. Or mebbe every other time you reload 'em. Or every third time. Experimentation will tell you this.
As to WHY they'd reccommend FL sizing for absolutely everything over .30, they don't give a reason, and I know of no blanket reason to do so. Have had Nosler's 5th ed for some time, and did not see that reccommendation.
(Snide remark
) Gee, I guess I'll have to start FL sizing the cases for my 8mm Mauser, and my .303Brit!
It would be nice if Nosler would explain why they put in that statement. General reccommendations like this normally come with a whole row of if's, and's, and but's.