Should it be expected that some champions will be viable at certain levels of play and not others, or is it reasonable to expect all champions to be fairly balanced at all levels of play?

I've heard a bunch of different discussions about champions being viable for low-elo but not high-elo, vice-versa, or putting mid-elo in the mix there. Because there is such a wide variety of champions, is it possible to balance all/most champions for all levels of play? I don't mean perfectly balanced, but to the point where the difference between the improved "trash tier" and the (possibly nerfed) "top tier" champions is relatively small.

Of course, it's easy to see how many champions could be balanced at this point, but there are some mechanics of champions that are easy to exploit at a lower elo, like stealth-based ones (although that mechanic is currently being reworked), or some more complicated mechanics, such as Twisted Fate's interesting abilities at higher levels of play.

Thoughts?