My colleagues from other disciplines always explain to me the very complex rules they use to determine (or better said, guess) the importance of the contribution of each author in a collaborative work based on the position of the author in the list of author names.
Myself, I don’t even try to do this. First of all because I don’t think their rules are consistent (some papers even devote the first footnote just to explain how to interpret the author ordering) but, even more importantly, my feeling is that in our community the order of authors does not convey such a strong meaning.
Maybe I’ve been lucky but in most of my papers the authors are just in alphabetical order since the collaboration was a real collaboration in which everybody equally contributed to the final result. The only exception I always follow is when the work is mainly the research result of a PhD student, in those cases, s/he appears as a first author.
What about you?
I think your approach is the most common one in our discipline (alpha, unless the work of a student or unless one person clearly contributed much more than the others).