I would be a little anal about the issue and argue that deadlifts from lower than "official" or standard height are more accurately called deficit deadlifts, and deadlifts from higher than that height are closer to rack pulls, which are a partial movement. These are both fine exercises, but they create additional complexity.
A higher start means you can lift more weight, since you're pulling through a reduced range of motion. The inverse is true of a lower start, of course. An additional concern with deficit deadlifts is the increased need for mobility and flexibility, without which you'll be starting the pull from a disadvantageous and dangerous rounded-back position.
I wouldn't argue that any of these starting positions is "optimal" in a truly objective and general sense, but the standard height is to be preferred because of its universality as well as its Goldilocks-style demand for "just enough" from both mobility and strength. Veering too far either higher or lower makes either the mobility requirements too rigorous or the strength requirements too weak.
If you are able to keep a safe, flat, athletic spinal position while pulling off the floor, and you're making progress, I say keep on going. You're doing a little more than standard deadlifters, and thereby getting a little stronger and demanding a little more mobility than them. That's awesome.