As the above poster puts it, the more people that know the higher the chance someone can rat you out to lessen their own problems.
Anyone who knows has you by the balls including the woman in your life. A sudden breakup in the relationship can be your undoing if she really wants to hurt you.
Nothing is ever fullproof either. If you grow and its illegal for you to do so, you are putting yourself at risk. You have to find everyway possible to minimize those risks and go out of your way to do it if you choose to grow.
You also may want to find out what levels of punishments are available for certain amounts of cultivation. There may be a cutoff for mandatory sentences or when the feds will get involved. Know this stuff before you make your decision and plan out everything as best you can well in advance from germination, growing, harvesting, curing and getting rid of it when its all done.
The most important thing to ask yourself is if the risks are worth the rewards.