I have to take back some of what I said in my other post, I have to agree with your method if it is a fixture that is hard to get to such as ball field lights up about 90', or any other hard to reach fixtures that labor would out weigh the cost, but for the most part I don't get that many ballast ever fail, and while I have replaced starters and caps, they are more likely to fail because of poor maintenance of people letting bad lamps cycle when there at their end of life, I try to explain to owners and managers that letting a lamp cycle for very long can cause the starter to go bad because of the cycling of the lamp so it is in their best intrest to replace the lamp asap, although the starter is a solid state device and tend to be either good or bad, a EOL lamp can cause it to fail.
Caps are another item that rarely fail, and these along with the ballast I have found that most fail because of either a lightning strike, over voltage or an unsecured cover allowing insects and or birds building nest in around them damaging the windings, I did have one that some one previously didn't re-install all the heat shields back in the fixture causing a lot of heat damage to the wiring and components to which I just replaced the whole fixture, as for the photo cells, that would depend upon how hard the fixture was to reach, and if the element was pointed toward the sun (in a southern direction) as they tend to fail more often then if pointed north (not always possible)