It is fully dependent on the location of the speaker. If you want to use a Q-Plug in the left front, you should also use it in the right front. However, it is perfectly acceptable (and can be advisable depending on placement) to use a Q-Plug in the center and leave the left/rights as stock. If your rears are close to a wall, use them.
We purchased a large amount of these in order to bring the costs way down so that we can offer them to you for a very reasonable price. With that in mind, my plan was such that for a few extra dollars, you can experiment with them and determine what configuration sounds best to you. They are an inexpensive tweak (especially for audio ) that can make significant improvements in your in-room bass response.
In my home, I enjoy the Sierra-1 with Q-Plug B when I run them full range (no subwoofer).