For the prairie restoration work in Indiana, the seed collection season runs from April to November, accumulating the harvested seeds during those months and sowing them as mixes by hydric zone/community type in December/January.
Generally, cold-moist stratification (overwintering) alleviates dormancy in prairie species and cues seeds that spring is coming, so it is a less risky time to germinate and get growing. For that reason, sowing seeds in December/January exposes the seeds to a few months of winter conditions and sets them up for germination in early spring. That's good.
However, we observed a poor conversion rate (the amount of seeds sown vs. the quantity of plants showing up in the restorations as a result) in the species which flower and ripen early in the year (April, May June) or what Chris refers to as ephemerals in his post above. We wanted to test the idea that keeping these seeds in storage during the summer months, instead of outside on the soil surface where they would be naturally, was causing the low conversion rates, i.e. limiting the establishment success of these species from seed. In the experimental plots, when we sowed seeds for 7 early-season species in the summer (closer to their natural dispersal season) there was greater diversity and shorter time to establishment of those species than in the plots that were sown in winter. This suggests that as best as your operations will allow, sow seeds in line with their natural cycles. I also encourage you to do your own trials and experiments.
Frischie, S. L. and Rowe, H. I. (2012), Replicating Life Cycle of Early-Maturing Species in the Timing of Restoration Seeding Improves Establishment and Community Diversity. Restoration Ecology, 20: 188–193. doi:10.1111/j.1526-100X.2010.00770.x