Generally best to use two pures, unless you're using two low-tier hybrids that are of the a similar type such as a Forest-Meadows with either a Meadows-Forest or Forest-Meadows.
The problem with using hybrids anywhere above commonly available bees is that they can contaminate results. For example, breeding two Forest-Commons together can result in a pure Forest, a pure Common, or move up a tier into Cultivated, not to mention any combination of aforementioned results.
EDIT: And due to the above reason about contaminating results, I confused Cultivated bees with Diligents thanks to countless attempts trying to get said Cultivated breeds by breeding Cultivated-Common with Common-X.
Hooray!