In thepory yes, after resizing is the best time to trim the cases. It is just that there isn't a lot of difference, and not a lot wrong with doing it the other way.
Cases are much more likely to contract rather than expand in length, by firing alone. It is primarily forming them back to an easy fit in the chamber that lengthens them. But it generally takes a few firings to do this to a harmless extent, and doing it before the last of them does little harm.
Basically firing thins (and therefore increases the sheet area of) the cases even where they are thick. Sizing has to squash them back again, only partly by thickening the brass, and it does it where they are most sizeable, i.e where they are thinnest and most annealed. The part of that process which isn't thickening of the brass, turns into added neck length.
Not all cases are alike in what happens. An old-fashioned one, with a very tapered body and thin walls, has a powder chamber which tapers from head to shoulder. There is a pronounced tendency for the pressure on that slope to stretch the brass forward, producing thinning near the head. A modern case, nearly straight-bodied and with strongly tapered wall thickness, tapers internally from the shoulder rearward. The tendency is for brass to be squeezed rearward.
This has two useful effects. The modern case has less tendency to separate near the head. There is also less tendency for it to lengthen. Finally, precision reloaders such as benchresters load to quite high pressures, but have their chambers and reloading dies very nearly identical in size, and they resize the neck only enough to give a very light hold on the bullet. They often use very few cases for a long period with no trimming at all. Note, though, that someone who hunts with cartridges in his pocket couldn't risk either of these, for fear of shifting bullets (dangerous if it is inwards), or missing the shot of a lifetime because dirt prevents the cartridge from chambering.
There is one more advantage of trimming your cases with the primers removed. You can use very inexpensive trimming devices such as the Lee hand case length gauge and cutter, aligned by a pilot ending in a rod which protrudes through the flash-hole. Even by hand this devices is fast enough to do this occasional job for many rifle reloaders.
Cases are much more likely to contract rather than expand in length, by firing alone. It is primarily forming them back to an easy fit in the chamber that lengthens them. But it generally takes a few firings to do this to a harmless extent, and doing it before the last of them does little harm.
Basically firing thins (and therefore increases the sheet area of) the cases even where they are thick. Sizing has to squash them back again, only partly by thickening the brass, and it does it where they are most sizeable, i.e where they are thinnest and most annealed. The part of that process which isn't thickening of the brass, turns into added neck length.
Not all cases are alike in what happens. An old-fashioned one, with a very tapered body and thin walls, has a powder chamber which tapers from head to shoulder. There is a pronounced tendency for the pressure on that slope to stretch the brass forward, producing thinning near the head. A modern case, nearly straight-bodied and with strongly tapered wall thickness, tapers internally from the shoulder rearward. The tendency is for brass to be squeezed rearward.
This has two useful effects. The modern case has less tendency to separate near the head. There is also less tendency for it to lengthen. Finally, precision reloaders such as benchresters load to quite high pressures, but have their chambers and reloading dies very nearly identical in size, and they resize the neck only enough to give a very light hold on the bullet. They often use very few cases for a long period with no trimming at all. Note, though, that someone who hunts with cartridges in his pocket couldn't risk either of these, for fear of shifting bullets (dangerous if it is inwards), or missing the shot of a lifetime because dirt prevents the cartridge from chambering.
There is one more advantage of trimming your cases with the primers removed. You can use very inexpensive trimming devices such as the Lee hand case length gauge and cutter, aligned by a pilot ending in a rod which protrudes through the flash-hole. Even by hand this devices is fast enough to do this occasional job for many rifle reloaders.
