that's all gonna depend on what type of vehicle you got too.
Let me tell you a brief story of what i recently did-
I bought a meter, i own a hatchback.
Typically, sub up port back is the way to go...
But, there are limits-
If running a lot of power\drivers, there will be a lot of potential rattle because port is facing the rear.
If box is very tall, it may end up weakening the glue that holds the glass together if the drivers are close to it or u may get lucky and just crack it, hehe.
So, i tested 24 different boxes in my car.
I found out for what extra room i could save for equipment and the best output combined, drivers firing forward with port firing toward the passenger side is loudest to the DRIVER.
This is not for comp, just for daily driving.
So, you gotta look at the +s on this-
(of course there is sound deadening regardless of which way you face your equipment)
Firing forward reduces rattle but does not mean it's recommended to use less deadening. Means it sounds better overall and does not annoy those around you.
Firing forward is cleaner sounding but only if the port is firing forward as well. However, output is compromised greatly. Firing to the side is like half clean, half boomy depending on note being played.
If you are overpowering your subs thermally, you will get the warning smell a lot sooner if firing forward.
I like firing forward with port to the side myself. It works well in my hatchback.
Less rattles, and better sound than just everything to the rear inside and out.