Defensive back rankings can be highly variable based upon your scoring system and even with that, there is a lot of year to year fluctuations especially at cornerback. If you read my DB drafting strategy article you know that my advice is to wait on drafting them.  Yes there are some elite players, but in general, the production from this position varies from week to week based upon who the player is going up against and what you expect the game flow to be. This is the most streamable position in IDP leagues, especially if the league separates out the safety and cornerback positions.  

The reliability of the linebacker position to get you points and the lack of depth at the defensive line position should lead you to drafting them first before going after defensive backs. The main exception is if you play in a very tackle heavy format where getting one the top safeties could be an advantage. Regardless, safeties remain the backbone of combined rankings as the volatility of corners on a week to week basis is what makes them streaming candidates.