I have a few rifles that have tight (fitted) necks (they wont take standard factory rounds)
The cheapest accuracy gain you can have when having a rfle rebarrelled is to have a tight neck chamber - it doesnt cost any more than a standard chamber.
The goal, is to have your brass sit entierly concentric within the chamber, any irregularities at the neck (uneven / non concentric neck wall thickness) will and does have e detrimental effect on accuracy.
Not that this matters much to a sporting rifle, but I for one prefer to take out all factors that will reduce accuracy, leaving only the nut behind the bolt as a factor of doubt.
How much to take off?
Entirely dependant on what the neck size is really - for example, if you have a 6.5mm with a .290" neck, you want to remove sufficient brass from the neck so that a loaded round measures .287 /8" or put another way the bullet is .264" diameter, the neck diemeter is .290" then the neck wall thickness needs to be .012" (264+12+12 = 288)
Is it worth doing?
Have to say yes, bear in mind you only ever do it once per case and its done.
Is it for everyone?
No, this is advanced handloading you need to be very careful and measure very precicely what your doing otherwise you end up with at best scrap brass or dangerously thin neck walls.
Dies?
Have to use bushing style dies for reloading and select a bushing .002" smaller than the loaded round diameter.
You can also neck turn cases for standard chambers, by doing a 3/4 turn clean up (clean the brass so that material is removed from 3/4 of the diameter of the case) this does also have a positive effect of accuracy.
Oh, I load for 22-250, 6.5x47, 6.5x284, 7mm/300WSM, 7mm/270WSM, 308, and 338LM - five of these are fitted necks.
I wouldnt do it if there was no benefit - it's not a fun task!
Hope this helps.