There's no use pretending to be something you're not—unless you honestly and truly want to become that person who you are pretending to be. I'm in full support of "faking it till you make it," as you are "faking" for the sake of yourself (to give yourself more confidence in a situation where you may be lacking in confidence). In that case, pretending is actually a manifestation of positive thinking.
On the other hand, being pretentious is a different matter. In this case, you are pretending to be a different person for the sake of others. Instead of channeling your insecurity into happy thoughts and actions, you block the possibility of genuinely making yourself feel more secure and confident by pretending to be (your perception of) some other "perfect" person. The grass is always greener on the other side. Trying to manipulate how others see you will only make you look desperate and uncool, two things which most people do not wish to be.
In short, playing pretend can work both ways. Only you can judge for yourself which will make you happiest.