While that's true both Ferricyanide and Thiocyanate can liberate free Cyanide under extremes. That won't happen with home disposal by dilution but does raise issues where spent solutions are carried in a vehicle - the issue here is in an accident where battery acid could come into contact with the chemicals liberating Cyanide gas. I came up against this at work and the regulations are quite strict, you need to be a licensed carrier (which we were).
Thanks Ian. Which is why I encourage everyone to read the MSDS for every chemical they are intending to use. Such information about dangerous decomposition products is included in this data. This information is easily available on the net.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.