According to the description, if I remember correctly, seeds planted together within "5 hours" should get the same quality when harvested.
Testing with this, I got three different qualities that were all very similar. 15/11/13, 15/10/13 and 16/10/13. This was done via manual planting the seeds within a minute or two of each other and different results were from the same stack of seeds even. When batch planting via the shift+rclick on seeds, they all seem to come out the same quality. Furthermore, the 5 hour concept doesn't seem to be functioning properly. I've planted seeds 30 minutes apart and consistently get significantly different results.
I'm not sure what's a bug, and what's a modification to the system if it hasn't been talked about publicly though.