I anneal after every firing of a case. The reasons for me are pretty simple.
1. It helps me keep consistent neck tension thus increasing accuracy.
2. The process really doesnt take that long. I have a set up that in about 15 min I can have 50 cases done. Considering I spend much more time in other case prep, whats 15 min?
3. I get to play with fire.
4. Its easier for me to track. I know that after everytime its fired it gets annealed so I dont have to try and guess or figure out how many firings it has been since the last time it got the heat.
5. I truly believe it really really extends my case life. Since I included this in my loading process I have yet to split a case.
6."Rich DeSimone, who holds the 5-shot IBS 1K world record, advocates frequent case-annealing: "On my 1,000-yard cases, I anneal every firing." As quoted from "6mmbr". I figure if you hold or held a world record in shooting you have some idea what you are doing. I am a copycat!
Hope that helps
The Art and Science of Annealing