Either way you're dead. No, do you prefere to be eaten alive or die from hunger?!
My point is it's easier to find some where that has food then some where that has ammo,and if you're against a bunch of zombies you're better off with ammo then a bunch of food-you can get by for a long time off the things in a common town.
And while Walmart generally being in an active area may mean more zombies,it also means more places near you that would have food,possibly even close enough that you could go back to your hiding spot on the roof,Walmart just seems like the best option no matter how I look at it.
You wouldn't die from hunger looking for food,unless a lot of others survived and had taken the food.
And then you definitely need to be in Walmart because there's no way you can arm more then a couple people without a gun store type place,also it isn't as important but Walmart has clothes and such so when it gets to be winter time you can survive on the roof easier.