I usually only do it when I consider I've been seriously inconvenienced. Some scenarios:
1, Recent flight from LUX to FRA to SIN (and onwards to BNE) - flight cancelled due to the plane being stuck in New York as there was a mechanical failure. Got in a day late so asked for (and got) compensation as per EU rules and I'd lost a night of prepaid accommodation
2, A few months before on the same flights - flight cancelled due to late arrival of my incoming LUX-FRA flight which had been due to bad weather in the morning. Again, a day late, but was refused compensation as weather is excluded. I disagree that weather delayed my flight as such (they had time to get a replacement and if Lufthansa can't find a new plane in Frankfurt, where can they) but it's not worth the effort sometimes.
3, In flight entertainment in business wasn't working for whole flight LAX - LHR. As I was upgraded for a pittance, I let this one slide as I enjoyed business class and watched some movies on my laptop when I wasn't reading or sleeping
4, FRA-MAD flight was delayed 3 hours - didn't bother asking for anything as I wasn't working in MAD until the next day so it was either 3 hours in the airport or 3 hours at the other end