The
comma before a
conjunction is actually somewhat arbitrary. Some
style manuals1 say "do it"; others "don't". And, a good
rule of thumb :
ignore whatever your teachers in high school said. That is, if they gave you some
absolute rule,
i.e. do or
don't.
In general, people tend to read one of the style manuals, agree, and run with it. But, there is still no universal rule (see: The Harvard Comma) despite what teachers might say.
The best thing to do is write for clarity. As long as a sentence has only one interpretation, do what "seems" best. There can, however, be a difference in meaning based on the comma; in which case, you need to be careful. For example, the sentence "I went to town with my parents, Ronald, and Nancy." If you leave out the comma then it implies "my parents' names are Ronald and Nancy" and with the comma implies "I went with a total of 4 people".
1 In case you didn't know, there are
quite a few style manuals:
The Chicago Style Manual and
The MLA Style Manual are the two I've used the most. There tends to be one for each major
academic division.
MLA is for
literature;
Chicago for
history,
&c. It's important to remember: these things are written by
people. A
group of
editors get together and discuss what they
think is the best
style. No one has the right answer;
nothing is set in stone. But,
Style is Style! Write beautiful.