who you are opening for is the most important thing....what is their style and choose records that will compliment them.

i remember one of my first times of dj'ing, i was billed as hard house on the flyer but because of the crowd, i changed up and started playing hard techno. well the guy that came on after me was billed as techno and so he got all pissed off at me cuz i was stealing some of his thunder and he came up and unceremoniously cut me off 10 minutes early....plus the promoter never hired me again...

so opening "properly" is totally important. it'll establish relationships that you need down the road as much as it a chance to show the crowd how dope your record collection is.