What difference would it make in terms of practical accuracy at the distance of 10yds and under that most individuals shoot at?
Trim or don't trim, it's up to the individual. Mentioned above is a quote that is just about as optional. I see many pictures of targets bragging on the great group shot by X gun with Y ammo at 5, 7 or 10 yards. I'm not saying that these aren't great groups shot by the individual but personally I wouldn't post some of them up as good shot at 50yds.
If you look at the rifle crowd they trim sort by weight and do all sorts of things to get their groups into as close to one hole as possible. Why, and it has already been touch upon above, should the same attention be paid to revolver or pistol rounds? Well in most circles a handgun simply isn't capable of shooting one hole groups, so why bother with exerting any more time than necessary to the production of the ammo, than to load it and shoot it, after all it wasn't designed to shoot a small group in the first place. Right?
I am not in any way for or fashion an excellent shooter, but I DO want the best that each of my firearms will deliver each time I pull the trigger. If that means trimming so be it. I so far have not trimmed the first piece of 9mm brass, but then again I have only had a 9mm pistol for about 8 months or so, and have yet to shoot up the initial ammo I loaded and purchased for it to see how it does. Just haven't had time. With my 45 ACP however I do know that it shoots its best with brass trimmed to .895 which is about the middle of the road. Will it shoot good with untrimmed cases, sure it will, but not quite as good. Case length determines pressure and the more equal that is, usually the more consistent the groups are. Right, wrong, or indifferent, it does make a difference. Groups at 5,7 or 10yds are fine for some, but I shoot at 10,15,20, 25, 50, or even 75-100 with my revolvers. Again I do not shoot great groups by my own standards or even half the shooters here, but I do take pride in hitting what I am aiming at when I draw down to do so.
All of my revolver brass is trimmed to equal length and matched by brand as is 95% of my pistol brass. It is simply something I do for no other reason than in my own peace of mind to make sure that things are as consistent as I can make it. I just feel that if I am taking the time to assemble the ammo myself in the first place, why not make it the best that I can from the ground up, or I might as well just by the cheapest crap on the market and be done with it.
Granted with range brass, or brass I have purchased as once fired, I do not get overly into the details as it is mostly used in places that I cannot recover it anyway. I do however still sort by brand, but in this particular instance the loads are used for trigger time and not considered being used in a possible life or death situation like most of the ammo I load. Most of it is either for hunting or SD, in either case I want to have the best I can when or if the time arises, and I practice with what I use the most.