The best thing about approaching women in the grocery stores is that their guard is down. They are not dressed up from head to toe and are in shopping mode.
Depending on what section of the grocery store you are in, just ask an opinion opener. Make sure you have some transition material otherwise you will end up talking about one or two things the entire time. And if there is no transition, there will really rough when you go for the number close.
Go for the number if you pick up some IOI's. I would also look for the social pressure, when asking for the number. If there are a lot of people around, she will feel embarrassed, awkward and could turn your offer down.
Hope that helps.
