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.