When you are first starting out, you should fold everything but monsters, and those you should raise. This is your safe place you can always retreat to. It yields something like -8 BB/100, so if you are doing worse than that, immediately change to this strategy.
Of course, raising a large percentage of the time can be better. Consider the case of raising some amount near 100% of the time. You win 1.5BB whenever you are not called, so profitability is a function of Villain calling frequency, Your bet size, and whether he's sensitive to changing bet sizes.... You could go nuts and map it out:
It basically says that the most profitable raises are ~4BB when the villain doesn't call a lot and is pretty sensitive to raise sizes, and ~2.5 BB when he calls a lot but is generally insensitive to how much you're raising (i.e. looking only at his cards). It also tells me that once his calling frequency reaches ~30% I should look for a new strategy.
I like raising about 75%-80%, and backing off only when it becomes unprofitable.