I am no expert but I would tee off to it.
Manifolds have a lot of internal features that not only cause pressure drop, but also are places where crud can build up/get stuck. If you keep it out of the main return line, you reduce your chances of having trouble with your return.
Tee off and if you have trouble with it your return will keep running happily regardless. Plus you can add a union and shutoff so you can remove the manifold for maintenance while still running your return.
That's my take anyway.
I would Tee off with a ball valve. You want to be able to control all aspects of flow to/through the manifold as well as the return. Water will take the path of least resistance, so you need valves to balance the flow rates the way you want.