There was a period where showing your midriff was not cute, but right now I say go for it!
We don't mean just baring that little slit of skin between your waiste line and your shirt hem, we mean fully bellybutton and all! It's summer, so you might as well take advantage and let a little skin show. I find the midriff to really be the most effective when least expected; during the day. And best with some toned down basics, like baggy, boyfriend jeans and Birks, I think the look is super cool.
Topshop at Nordstrom has a great selection of tasteful crop tops if you're looking to scoop one up.
Isn't it funny how your view on something can do a complete 180 in a matter of seasons? With style becoming more personal than ever it seems, there's not much that's off limits these days.