Collating isn't an exact science, that's for sure. In a perfect world it would take you 3 boxes + 2 packs to get 1 of every rare, but the laws of averaging would never allow that to happen. Here are my strange pulls, I've bought 56 packs of Invasion so far and gotten:
2 Urza's Rage
2 Meteor Storm
2 Ghitu Fire
0 Fact or Fiction
0 Shivan Oasis and Elfame Palace
5 Breath of Darigaaz
5 of that horrible black instant (2B, 1/1 and fear?)
2 of that green common: 2/1 CIP 1 mana of any color
1 Box was ordered from AZ, 20 packs from my hometown in CT.
My rares have been very balanced so far (1 of's have been good to me, Tsabo's Web, Skizzik, Rout, Rith, etc.) but my uncommons and commons have had very poor random collation.