As far as I hear Poloniex does not stake the POS coins (unfortunately, I forget and can't provide the source of the info), neither does BitTrex, I think, but better ask them directly  
I heard (in the Ambercoin thread) C-Cex does stake. And I am affraid Bter does the same.
For the coinholders it is better to trade their coins on the non-staking exchanges.