as long as you've covered every exposed part, and not just that big fat dollop that people love to only do in the middle. some cupcakes even taste better after sitting because the flavors marry...carrot or spice cakes are the first one that comes to mind like that!
sometimes things get crazy around my house, and i lose track of time. next thing i know, i have 60 or more cupcakes cooled off on my counter, it's bed time, and i don't have it in me to whip up a batch of icing. i've learned to make my icing first, and have it sitting and waiting, covered with a damp towel. when the cupcakes are cooled well, i just spread a thin layer, like a crumbcoat, over them. that way,e ven if i run out of time that day and can't decorate, they are protected until the next day when i do have time.
simple concept, but kinda took a while for me to incorporate!