No, you shouldn't need a FP+. Typically, by 8:30pm or so, there is no line for Safari. Several times we've literally walked right on it...the CMs were just waiting for people to show up and get on the buses...once there was only one other family on the bus with us.
But be prepared that in the dark, it's hard to see animals. There's some lighting, but not a ton. Still worth it if you've got the time.