After applying subsurf to my model it creates small N-gons in several spots causing “pinching” as seen below in the image. How can I avoid this? As the area shown is made up of quads, so I should be fine right? Not sure how to fix this one 
.blend: https://dl.dropboxusercontent.com/u/13532502/Slot%20Cars.blend
Image: https://dl.dropboxusercontent.com/u/13532502/subsurf.JPG
I believe thats caused by that top edge loop not being a continious one from front to back. Your creasing 2 edge loops instead of one and where they join, you get that result.
I did find that the loop going across to the left of the highlighted (orange) loop came to a dead end which I fixed, but it made no effect. Perhaps I don’t completely understand you as the creased edge (pink) goes from front to back, and the highlighted loop goes all the way across. You can make out the N-gon just below where the pink and orange loops meet below:
Could it have something to do with the second loop that goes around the wheel well? Does it need to run front to back?
Edit: What I have noticed is that these N-gons appear whenever 5 edges meet on one vert, so do I need to avoid this entirely? Even then, not really sure how this can be avoided.
If you alt click that edge, it will only select part of that loop. That means it is not one continuos loop. Youll notice that it is actually 3 seperate loops and it pinches at each point they meet.
Ah I see what you mean now, how do I unify the edge loop?
