I've been handloading since I was 12, so 45 years now. There are no hard and fast rules, you just have to consider the brass and the load.
As someone mentioned above , pistol brass and straight wall file brass can be damaged in shipping and have a number of cases that are out of round. Based on the percentage that are out of round and the overall condition of the brass I might size them all or I may just resize those that are out of round.
Even if the brass looks perfect it sometimes depends on what I intend to load it with. A .001" or .002" over size cast bullet will almost always have adequate neck tension, while a jacketed bullet may not.
With revolver loads you can roll crimp the bullet in the case to prevent the bullet from backing out under recoil, and initial sizing may not be necessary. Loading up a round or two and testing will tell you a lot more about what *you* need than asking on the internet. A heavy load in a light revolver will need more neck tension, a taper crimp or a roll crimp than a lighter load or a heavier load fired in a heavier revolver. If you plan to shoot the load in multiple revolvers test with the lightest weight revolver.
In contrast, with a semi auto round that headspaces on the mouth you are limited to a taper crimp and in that case sizing first will usually get you an undersized case with a wasp waist shaped profile that will prevent setback with a factory taper crimp. But once again you can load up a few rounds or dummy rounds and then load them from the magazine repeatedly to check for set back to see if an initial sizing is needed.
As someone also noted above, some brass arrives properly worked and ready to load, while other brass may need to be deburred and in some batches even trimmed to a consistent length. If the case is shaving lead or gilding metal off the bullet during seating you may need to deburr. Increasing the bell will solve the problem as well but it over works the case mouth and greatly shortens case life. With the exception of some soft or very sharp flat base cast bullets, if you are not actually crushing cases in the seating process, you have enough bell on the case.
If you are going to roll crimp the bullet, you need a consistent length to get a consistent roll crimp and if the brass isn't consistent in length, you need to trim it. Even if it is consistent, if you are going to mix that new brass into your general population of brass, you may need to trim it to a standard case length.
In general, straight wall brass doesn't grow in length appreciably over time (unless you are doing something strange or overloading the bejeezus out of it). I'll trim and deburr them when new and then run them until the neck splits, or in cases like the .45 Colt (and occasionally .38 Special) until they develop spider cracks in the side wall.
Short, straight wall cases like the .45 ACP that are not roll crimped will go forever unless you over bell the case mouth. Bell *just* enough to seat the bullet without crushing the case (less for jacketed bullets, a bit more for cast). Most folks bell way too much.
Similarly your revolver brass will last a lot longer if you don't roll crimp, or worse, over bell it and then roll crimp it. I reserve roll crimps for loads where:
- heavy loads and or light revolvers require a roll crimp to prevent the bullet from backing out under recoil and jamming the revolver; or
- for slow burning colloidal ball powders in magnum loads where I need a heavy roll crimp for better ignition and efficiency.
I generally don't use colloidal ball powders like 296 in my .357 magnum revolvers, particularly my shorter 2 1/2" and 3" revolvers as the heavy charge weight increases the recoil by about 40% compared to a faster burning powder like Unique and will only increase velocity maybe 25-50 fps. The juice just isn't worth the squeeze - or the significantly increased forcing cone erosion. A taper crimp is normally sufficient to prevent the bullet from backing out.
I will however use a slow burning colloidal ball powder like 296 in my .357 Magnum carbine and rifle loads where the velocity gain from the slower burning powder is much more pronounced. In this case a roll crimp isn't needed to prevent the bullet from backing out, but it does improve consistency and reduce the standard deviation in velocity.
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In short, there isn't a single right or wrong answer, you just have to consider the brass on hands how you plan to use it and whether it's worth the effort or not.
That differs as well. Once I switched to a Dillon 550B about 20 years ago, doing an initial sizing was zero extra effort compared to using a single stage or turret press. The question then was just whether to trim and/or deburr.
