I have heard those the copper in those snail killers is enough to kill your plants as well. Bleaching will damage or kill your plants and won't work for some snail species (your plants will die before the MTS probably will). Pond snails, ramhorns, and MTS do not eat healthy plants. I think the copper gets into the silicon of the tank so it stays forever but I am not positive on that.
Assassin snails only eat other snails of a certain size (around their own size)
http://arizonainverts.com/forums/index.php?showtopic=204
The link above has a lot of good info on assassin snails.
Love that article. To wash your plants you might also want to look into using potassium permanganate (KMnO4) http://www.fishpondinfo.com/plants/algaeinfo.htm#clean look under "Ornament and Plant Cleaning"