To blowhole or not?

It really depends on the air flow of the case (front to back, or bottom to top) and the amount of air going in vs. the amount of air going out, because simply adding fans all over the place can be more harmful than beneficiary (don't want turbulence inside your case). Ideally, you want to exhaust a similar amount of air as what goes in. Of course, you also have to keep in mind the "stock" airflow capabilities of your case; if it has little holes on it (many newer cases already have little breathing holes to facilitate airflow, usually on the back or next to where the expansion slots on the MB would be) you are better off with pumping in more air than what goes out (to maintain positive pressure inside the case and avoid dust build-up).
In your case, if you're going to add a 12cm intake on the side panel, a blowhole on the top will be ideal. Those two 8cm exhausts on the back won't be able to get rid of all the air that the two intake fans will pump in, air that will eventually heat up, and if your case is of the older whole block type cases (the ones that are completely closed off except for the intake and exhaust fan ports), your components won't be cooled as effectively. In this case an extra exhaust (preferably at the top) will be of great help.

Happy Modding !!! Hope you get good presents .
Having a top blowhole is also good since it forces air upwards, therefore cooling the upper part of the case (the top of your MB, PSU, any drives you may have). Back exhausts usually end up near the middle of the case, forcing air out before it raises to the top. It would be better if the 2 8cm fans you have in the back are low speed fans while the top 12cm fan does most of the "blowing".
My case has 2x8cm intakes and 2x8cm exhausts stock. I added 2x12cm intakes to the side panel (one in the center, the other near the expansion card area) and 2x12cm blowholes on the top. I can actually feel the hot air coming out from the top on hot summer days.