[kaigoss69]

That depends. If you like a lot of sound coming from the rears, I'd say upgrade them. However, if you are mainly concerned with the front sound stage (which the rears detract from), then you can either leave them out or just fade them in a tad (in which case the stockers would be fine).

Generally, people who listen to EDM or who prefer the "whole car sound" want to have the rears play as loud as the fronts. People who are concerned mainly with staging and imaging (having the singer and/or instruments "placed" on a stage in front of you, which means listening to the music the way it was recorded), generally do not want much (or any) information coming from the rears.