You are going to get way more PP for playing harder songs. But, you will get PP for every song you play, no matter what. There is something called a weighting system, though, which makes less PP plays worth even less. For example, if you have a couple really good plays, like 88pp and 85pp, then you get an 82pp play. You're only going to get 90% of the PP for that play, if it is your third highest play. Therefore, you will get 73.8pp for your 82pp play.