Here's the thing...
It is true that riding with your seat lower than the optimal height can do a few things. It overworks your quads and glutes. It can give your knees a bit of trouble. It is not as efficient.
BUT....
If you don't ride at all because you are not confident to ride at the optimal height, then that is a big shame. So if it is not giving your knees, quads, glutes etc any trouble, and you don't care if you are not quite as efficient as you could be, then I reckon you should ride at whatever height you feel comfortable with for now!
I'm sure in time you will gain confidence and speed and distance and you will want to be in a more efficient position (and aches and pains may even dictate it to you) and then the seat can go up a bit more. Do what feels right for you.
2008 Shogun Ninja/BBB Women's Race
2010 Scott Speedster S20FB/BBB Women's Race
2011 Avanti Vitale 3 (converted to flat bar with triple)/Zero Zia Pro
2008 Kona Lisa HT/WTB Speed She
2009 Specialized Era Marathon/Ariel SL 143
2008 Holstar Roadster (tandem)/WTB Speed She