I think I would be wary, but not for any rational reason. From a rational perspective, of course she'll be fine. I also have a very sensible 9 year old that I'd leave to walk to the shop, but would be more wary driving, which I know is ridiculous as I'm just as likely to be run down as a pedestrian by a stupid driver as I am in a car...and in a car I'm safer.
I think you have to go with your heart on this one. I do leave my four at home for a few minutes to walk to the shop. They have the tv on so they're not likely to go off and do other, more dangerous things, I call my mum or my closest friend to make sure they're around at the end of the phone in case any of the kids are worried about anything, and then call them when I get back to tell them I'm home safely.
My kids are all really sensible, though, and I am more scared of other people's nosiness and nastiness than of anything actually happening to them.
And to the poster who said she was 'done for neglect' - I think 90 minutes is another matter entirely, and it is not being 'done for neglect' to have a few concerned phone calls and letters.
I once left my kids in the car outside staples while I ran in for some glue. Came out five minutes later to see a policeman waiting outside to tell me off...but that's all he did do. He didn't caution me or put a child protection order in place or anything - he just said someone had called the police, and that he had to come, obviously, and maybe I shouldn't have done it.
I felt like shit afterwards, but I still knew in my heart of hearts that they were all pretty safe once I'd done a mini 'risk assessment' whereby the risk of my youngest running out of the shop into the car park because she didn't want to be in the shop was greater than the risk of anything happening to her in the car, strapped in, and with her sensible older siblings.
The trouble is a lot of parents don't make considered 'risk assessments' and don't put contingency plans in place, and they 'spoil' it for those of us who do, making every assume parents who don't wrap their kids up in cotton wool are automatically neglectful.