A lot of good info above.
Set up the process that matches your equipment, and do the same every time. Consistency is what I am after. Does any one operation drastically affect accuracy? Most likely not. However if you don't do each step every time, you will start to see variation, and won't know how to fix it. Also, it depends on how far down the rabbit hole you care to travel.
That said, I anneal, size, trim, chamfer in and out, brush necks, and clean primer pockets, each firing. I have invested money in motorized equipment to make the process fast and accurate, so it takes about 1.5hr to fully process 100 rounds of brass, including wait times.
1. I trim enough to leave .010" to end of chamber (as close as I can measure). chamber lengths can vary quite a bit from Min to max SAAMI not even accounting for custom chambers. BUT, you must trim enough for the neck to expand, or you can over pressure your firearm, which is dangerous (thus SAAMI specs). Proceed carefully and with knowledge with custom chambers.
2. Small cases (6BR, Grendel, etc.) don't grow much (.0005" +/-). larger cases can grow over .001" per firing.
3. Trim length variation; as close as my equipment allows (depends on how good a job I do on sizing, as trim is based off of shoulder). Usually within .001" overall case length.
4. Chamfer very little; usually 1 to 1.5 revolution with light touch against motorized cutters (just enough to insure the sharp edge is off of ID and OD).
End of the day, you most likely will not see an appreciable difference shot to shot if you do it all or none if you are not applying precision to the rest of your processes.