There is a bit of a compromise in getting the fit sometimes, and brass fired in an oversize chamber may need a bit more, due to the memory springback. Still, for me, when sizing scrounged brass from several unknown sources, I FL resize all and start from scratch fire-forming in the intended gun. I do tend to keep brass segregated per rifle too. This is what I do:
I set the FL resize die down to the raised shellholder, just a comfortable snug on top and finger tighten the lockring and set the screw, then I back it off (usually) 1 turn.
Size a case that doesnt fit into my chamber very well and test the fit, usually it still wont go, so I turn the die down about 1/6 turn, and repeat until it fits with no resistance (BTW, make sure the 'trim to' length is OK before this, an overly long case can really mess you up).
Once I find the die setting that 'seems to work' I do several more, but some fine tuning is sometimes needed; then, in my reloading book for that rifle and cartridge, I make note of it.
This way, if you have several rifles with the same cartridge, you can get quick repeatability when needed, and if one really needs the FL resize you are all set to run the die all the way down to the lockring.
I have found, too, that shellholders from different companies are not dimensioned all the same. Some are a little 'thinner' on the top than others and give you a few more thou. sizing, others a little thicker. On one 'very closely chambered' rifle I had to chuck the SH in the lathe and grind the contact surface about .007 thinner to get the FL resizing it required. Also, if some brass is grossly oversize, or you are reforming from another cal., say 30-06 to 25-06, you may get enough press spring to actually not have contact to the SH, you have to get down eye level at that contact point to see it happen on hard to resize cases.