Unquestionably, as you have heard, your SSD should be connected to your motherboard's 6 Gb/s connector (sometimes referred to as SATA 3 or SATA III) usually designated as a SATA 1 connector, however, I see no downside in connecting your other drive, the HDD, which apparently will serve as a storage/backup device to another one of the motherboard's 6 Gb/s connectors as long as there's a vacant SATA 6 Gb/s connector to do so. While this won't have any effect on the performance of that drive there are no negative implications affecting your boot drive that I'm aware of. I would agree that there's no critical need to do so and if so choose you can leave well enough alone should the HDD be already connected to a SATA 3 Gb/s connector.