Any anemone that is to be cut should be cut right down the middle. If you are a little off center, no worries. They will heal fine as long as your prop system and methods are adequate.
As far as releasing toxins into the system. Yes, any anemone that has been cut will release some nasty stuff for a while after being cut. It is best to have a completely different system for anemones that will be cut. If you have to return them to a system that contains other livestock, you <u>must</u> put them in a seperate holding container with aged water from the tank it came from for at least 1-2 hours after cutting before returning it to the main display. This will allow the anemone to release it's nastieness outside of your main display.