Hmm. Good question. I think as long as you know she can be 'trusted' around the house without getting into trouble, then any age is ok. Just be sure that your floor, or any other places she can get to easily, is free from things that may be dangerous, such as toys that require supervision, etc.
I got Stig when he was 5 months and let him roam free in the kitchen/dining area during the day when no one's home at 8 months. We leave him a clean bowl of water plus some very durable toys to play with. We also leave the stereo on low volume. He seems to be sleeping for most of the day too.
Spaying is done commonly between 6-8 months I think. But your vet will let you know when it's the best time, taking her health into consideration.