Temperatures need to be taken on first waking, so a 7:30 temperature when you've already been up for a while is going to be useless.

Taking them at the same time every day is best, but if you really can't face a 4:30 am wakeup on the weekends, just go with your weekend temperatures being at a different time. You may find that it doesn't have much of an effect on the overall chart, but if you find that it does, you've got plenty of time between now and then to figure out how to go back to sleep after taking your 4:30 temp on the weekends.

Good luck with the charting, and good luck with the TTC when it starts!

BW