The rule is: put a comma before an 'and' when you are talking about a list of things - especially with multiple words. It is only optional if the words are singular: gum, gin(,) and rummy. But piles of work, eating too much, and getting tired. The first sentence is optional to have the comma - the second not.

Example "Go to 'Offers', fill in the doc, and click 'Save and Generate'" Yes, the comma before "and" is required because there is a definite pause there. When I speak that phrase to myself, I insert a little pause after doc before saying the rest of the sentence. The comma I place before "and" represents this little pause. Adding the pause or comma after "doc" emphasizes a little more that you must fill in the doc. If you leave out the "," then you are saying that filling in the doc and clicking save is more like one action only.