Most SQ installs have the front stage (usually the L & R front speakers) pointed towards the center dome light or a headrest (biased towards one front listener or the other). Most accomplish this by using the Q Logic kick panel install shells which move the door speaker down into the foot well and points them up and in. If you are going to this extreme, you will likely be running components which have a separate tweeter. Some like the sound of the these mounted down with the main driver, some like them mounted high in the sail panels. It varies by speaker, vehicle and listener preference. The best way to experiment with speaker location is to use a bath town bunched up in a ball and bury the back of the speaker in it on the front floor so you can move them around the angle the placement while listening. Use Velcro on the tweets to try different locations. BTW,if you go with the Q Logic lick panels, they will need some reinforcement, they are fairly light and the plastic doesn't sound great until you reinforce it a bit.
Any multi channel setup in a vehicle is more for the experience of installing it and trying it out but honestly, a good processor with a phantom center channel will give you close to the same experience and maybe better. It depends if you are able to find a good location for the center channel speaker or have to settle for a dash top mount shooting at the windshield. Also, if the center channel speaker does not have similar characteristics to the others, it will sound off. There are a lot of variables.
The sub box can be sealed or ported. Sealed does not require an open port for "the sound to come out". Sealed is the easiest to build, and can be smaller than ported. Most average subs will need anywhere from .8-1.5 cubic feet of air space to work properly in a sealed box.