I'd leave the house in a reasonable state in case you need to get back in touch with them. You never know - something could be inadvertently delivered to your old address, you could have left something behind... you don't want it to be embarrassing.
I don't think people would expect you to leave the garden looking like Alan Titchmarsh lived there, but give the lawn a final mow and keep on top of the weeding as you'd do if you were still staying.
That said, the people we sold our house to insisted we move out 10 days before the rest of the chain was ready, leaving us homeless and our stuff in storage during that time. It cost a fortune. At that point I decided they could go jump and didn't do half the last minute stuff I'd intended.