I find top platforms front and back to be extremely useful - especially in back if you have a rack with dual rails. Baskets like
these from Wald make them more useful, and with zip ties/hose clamps/velcro straps are easy enough to put on and take off when you need to. I have a Tubus Cosmo along with a Wald basket on the back of one of my regular commuting/touring bikes, and a Nitto rack with platform up front that sometimes has a basket on it as well - I can carry two panniers worth of groceries, a twelve pack in the basket on top of the rear rack, and a loaf of bread, the newspaper, and a bottle of wine in the basket up front.