First of all, let me correct a misapprehension about the Wiccan Rede. The Rede is advice, not a law. And what it says is that IF your actions will not cause harm you are free to do them. The Rede is absolutely silent on those actions that might cause harm, leaving the decisions regarding these actions to the ethics and situation of the individual Wiccan.
There is no single opinon or rule within Wicca in regards to suicide. Whether suicide is appropriate or not would, in my opinion, depend upon the situation in which the individual finds themselves.
For instance, if the individual had been diagnosed with a terminal disease and was experiencing unbearable pain that could not be relived by any available treatments then suicide might indeed be a valid choice for the person. As a Wiccan High Priestess, I would support any of my coveners who found themselves in this situation and made this choice.
On the other hand, if the individual were in good health and were contemplating suicide because they were depressed or they found themselves in a difficult life situation that could be remedied and was only temporary, then I would try to help them find a less permanent solution to a temporary situation.
Remember that just because we are Wiccan it doesn't mean that our lives will always be perfect. Sometimes the Gods give us difficult and painful lessons to learn. Suicide is a way of cheating and failing to master these lessons. When that happens it is likely that you will simply have to face similar lessons in the next lifetime.