If you are talking about routing a roundover on the front of the baffle in that middle open portion, I would probably assemble first and roundover that entire circumference of the front baffle edge after assembly. If you are talking about routing the rear facing edge, I don't think I would, I'd leave that edge as is. The only benefit I could see to routing that middle piece before assembly is if you wanted to leave the short 3/4" rise with a square edge.
An alternative consideration would be to glue a 3/4" strip across that open gap, to bring the top edges flush across the entire front face. It could be 3/4 × 3/4 or even 3/4 tall x 1.5 deep, with a small or intermittent corner wedge brace hidden on back side to support it. That would give you a continual clean line all the way around the baffle and would be my preference. The idea of adding that 3/4x 1.5 would be just for appearance, similar to face frame on a bookcase, adding rigidity and visual substance. The front open baffle would appear 1.5 just like the x-static., but adding a 3/4×3/4 would also look great.
Good luck,
Elon
These are thoughts only. Whatever you decide to do will work fine.
Edit: after seeing the photo below, another option if you have the tools for it would be to add a hardwood baffle on top of your existing cabinet. The model below is a veneer. You could certainly build yours as a double layered baffle, laying the new over the existing. If doing so, you'd need to enlarge the driver openings on the mdf so that the back end of the drivers have plenty of breathing room . There are some considerations to be aware of when incorporating solid wood materials but in my opinion a solid wood baffle is less labor to final finish than painting mdf.