If you leave at 6:30 a.m., by the time you reach the outskirts of Houston and inside, the rush hour traffic will have dissipated. Traffic flow would be in that direction -- inbound -- but you'll be approaching from the southwest and exiting 59 for the Sam Houston Tollroad both geographically before you would be susceptible to hit a lot of congestion and at a time of the morning after you would be likely to do so. It's up to you, of course, when you want to leave, but as someone who spent 18+ years of my life in Houston, who has driven all over it at all hours of the day including rush hour extensively, and has made countless Hertz drops at HOU, I think a 6:30 a.m. departure would make for a comfortable drive in, with some margin for error, unless you drive well below the speed limit along the way. If you leave at 5:00 a.m., on the other hand, you may get to sample some rush hour traffic in Houston.