Theres no such thing as overkill. Because everyone has their own definition of overkill. To some anything more than TV speakers is overkill, while others have Danley tapped horns in their living room. Now I wouldn't blow my entire budget just to be able to add towers as surrounds or skip on quality fronts just to be able to have towers all around but if you have an EMP system for example picking up a pair of E55Tis when they are on sale at crazy low levels isn't bad.
Owning a stereo pair of the new Pioneer FS52 towers as fronts but then later down the road spending another $200 for another set as surrounds isn't a big deal, consider the small matching bookshelf and a decent stand could cost more.
Theres no standard, its whatever your happy with go for it.
I had a co-work several years ago who just loved his system and talked about it all the time. He uses Bose 161 bookshelfs, Bose center, and Boston VR3 towers as surrounds, the most oddball backwards system i've ever seen but he likes it so thats all that matters.