There is, indeed, no widely agreed-upon convention. I prefer writing Concentric CSS where I list the properties in order from the outside of the box to its inside, so that I can remember whether the padding goes inside or outside the borders, and so forth. After reading your excellent question here, I realized that I felt strong enough about it to write a blog post, so here you are in case you're curious:
http://rhodesmill.org/brandon/2011/concentric-css/
Note that the popular Box Model Convention gets the order wrong in several cases. The actual CSS rendering goes in this order, from outside to inside:
+-------------------+
| |
| margin |
| |
| +---border----+ |
| | | |
| |(background | |
| | color) | |
| | | |
| | padding | |
| | | |
| | +-------+ | |
| | | height| | |
| | | × | | |
| | | width | | |
| | +-------+ | |
| +-------------+ |
+-------------------+
Which suggests a natural ordering for your CSS:
margin / border / background / padding / height × width
The "Box Model Convention" instead uses this rather bizarre order:
height × width / margin / padding / border / background