Well, at least one of the posters here does it pretty much the same way.

I basically have two categories of Catapults, "Wall Breakers" and "Defender Softeners". The Breakers get accuracy promotions and are usually built in cities which don't specialize in military, because they don't need to have a lot of experience. the Softeners get built in the cities where I have my XP bonuses, they get as many City Raider promotions as possible, and then either Combat or Drill promotions, depending on the expected strength of the opposition. Some units may break this rule and get Accuracy after the three City Raider promotions; these are for flexibility, but they only get built when I have enough Softeners already, so that I have the best chances on the initial softening attacks.
Unless I'm pressed for speed, I also attack with all siege units that have a survival chance of 96% and higher. Once the initial defense units have been wounded, this is often a very low-risk way to gain experience on the siege units, and make them stronger.
I have no idea whether this is the best strategy (I actually never look into the strategy forum since the game keeps fresh longer for me if I develop strategies mostly by myself), but it always felt right to do it this way.