Im not a huge fan of things that are made to only look like something functional,but are really cosmetic. Unless you are going to make them functional,and that functionality would make a legitimate difference to your vehicles performance (cooling,cool air for the intake,etc) then I personally would just skip it. In other words,vents and hood scoops should actually duct air somewhere useful.
Its not nearly as bad as some things though. For instance, dual exhausts should actually connect to two different banks of cylinders, if the end of your exhaust pipe is 3" in diameter it probably should be 3" on the other side of the muffler and all the way back,and you probably shouldn't bolt it up to the sad little flange on the end of that stock exhaust manifold. Most importantly though,what I really hate to see,even worse than a Honda Fit with a big chrome muffler hanging down,is a Geo Metro with a 3" exhaust tip that necks down to 1-1/2" where it attaches to the muffler. (No ones going to beleive you have three inch pipes and a set of headers on that 3-banger)