The better you know someone, the less need there is for safe words. If you are acting out a scene with someone you are not very intimate with, safe words are absolutely essential. After all, we play in an environment where 'no' often means 'yes' and where 'please stop' usually means just the opposite.

I like to use 2 levels of safewords, a red light & a yellow light if you will. The yellow light is something that might actually be said in the context of the scene (mercy, perhaps). It means "I don't want to stop, but I need a break or change". The red light is something that would not be said as part of the scene (parsnips? rutabaga?). It means "You must stop right now!" & must be honored.