Here is an old trick I had forgotten about and just got me out of trouble again, using Structural Beam systems for picket fencing.
Just make your picket in a Structural Framing - Beams and Braces family template, then use this as your beam in a Structural Beam System. (Structure tab/Beam System).
Draw yourself a referance plane (named) where you want your fence then set your work plane as this named ref.plane. draw any shape you like, even splines. If you pickets end up upside down pick the opposite side as Beam Direction.