Well, I haven't had Bobby's lucjk when it comes to trimming. I have to trim about every third or fourth firing, depending on how heavy the load was in each case. But then, after neck-sizing, charging, and seating, I crimp most of my 22-250 cartridges using the Lee factory crimp die. Some are a light crimp, others are heavy. I do it because I've noticed an improvement in accuracy, but the truth is that I don;t have enough rounds through this rifle (about 400+) to have really tested this crimping. In order to be accurate, a particular load must be shot several times with results accurately recorded, then crimped, and shot several times again with results recorded. I haven't fired enough yet in this one, I've just noticed some very tight groups, whether crimped or not, and record them as such.
The best thing to do after each firing is: once your brass is clean and then sized (full or neck), check each case with calipers. Once you get familiar with your trimming tools, you'll begin to know when a measurement is close enough to trim or whether you want to leave it alone for another firing or two. Just because a case has exceeded the length of your trim gauge does not necessarily mean you must trim it. Sometimes I'll let a few thousandths on a case go for another firing before trimming or, before re-echecking the length anyway. But...when it begins to get close to the maximum case length, you must trim it.
I full-length size new brass and every time after annealing. In-between, I neck size only. There has been a time with my 22-250 that I had to full-length size even though the brass wasn't ready for annealing. At the range, one cartridge out of twenty wouldn;t chamber, so when I got home and prepped all the brass, I went ahead and full-length sized them all (even though all but one chambered just fine). Better to just do them all, I figured. If I remember right, the brass was on it's 3rd firing, so two firings later I annealed, and full-length sized them all again.
This annealing must be done correctly. That's important. Read as much as you can on the subject or you could end up 1) doing nothing worthwhile, 2) ruining your brass, or 3) creating a very volatile and dangerous situation in your rifle. I'm no expert at annealing (or reloading for that matter - I call myself a "yearling"), but I've read all I can in this forum and others, and have done it many times. I've also had a very bad experience at the range back in July with my 30-06. I think the problem was due to improper annealing.
When I load a batch, the type of bullet, type of powder, charge weight, primer, overall length, and whether or not the bullet was crimped is all recorded. Also, for that batch, the brass is recorded. I do it with a two-number recording like this: new brass is 0-1, meaning it has never been annealed ("0") and it will be, when fired, fired once ("1"). After four firings, the loaded cartridges are recorded as 0-5, meaning this batch of brass has not been annealed and, after firing, it will have been 5 firings. Then I anneal, full-length size, and the next time that batch is loaded into cartridges the brass is recorded as 1-1. Make sense? Probably not....only to me, maybe.