Brass trimming question

[Nanook]

I just bought a .30-06 rifle, first time for me with this caliber. Most of my reloading experience is with handgun ammo on a Dillon 550, with some rifle on an RCBS single stage.
My question is this: in a bolt action rifle, is it necessary to trim after every firing? Or as I've heard does the brass more or less 'fire-form' to your chamber?
Just curious, I'm in the process of trimming some once-fired brass given to me by a friend and I'm wondering if it's necessary all the time.
Nothing exotic, just 150-165 grain loads.

 

[capbuster]

Your maximum case length on the 30/06 is 2.494".If your cases exceed this, you should trim to the length of 2.484".This information comes from the Lyman cast bullet handbook.As with all your reloaded rounds you are striving for consistency so the same goes for the length on your rifle cases.

 

[Nanook]

Capbuster, are you the same capbuster from Loadyourown.com?
I understand about the lengths, I was curious about the fire-forming question. What do you think about this, is it fact or fiction?

I'm trimming the brass to the .2484" dimension right now. Quite a bit of it was near the 2.494" max, in fact some of it was over the max.

 

[braindead0]

Used to shoot a lot of 30-06, generally speaking I probably only ever checked the cases every 10 reloadings (more often on full power loads). I suspect though that my dads old springfield would eat just about anything ;-)