I've personally never bothered with the DFP passbacks.
I much prefer to use the "Daisy Chain" method, where you just chain networks together to build out your waterfall. In theory its probably "better" in some fashion to do this within DFP (and you could argue that its senseless and should just invest time in Header Bidding) but it certainly does work to use a Daisy Chain.
Here you trigger the first network in your chain (Could be Sovrn) and then each network passes to the next when they don't fill the impression. You might have Sovrn-OpenX-Rubicon-Adsense - putting AdSense at the end ensures you fill everything.
By no means is a Daisy chain the best way to target ads, but its a technique which has certainly worked since the beginning. I've always found the daisy chain easier to wrap my head around, and even when we've moved to header bidding for most sites there's always a few impressions at the bottom we "mop up" with a small chain.