It's a judgment call, of course.
The advantage of assembly in the shop is you don't need to work on the ground outside and your assembly tools and fasteners don't need to be carted to the site. Then for the install, you only need to send out a couple of digger drones with a level. If your truck is loaded with tools and screwa, of course, and you have 6 of these signs to do, it may be easier to load up posts and panels and assemble on site.
TIP: screw a piece of pointed angle to the ends of each post so that it sticks out at the bottom. When you set the signs, push the pointy angles into the dirt at the bottom of the holes. You can plumb and level easily, even by yourself—and without braces. I have a drawing of this but I couldn't find it.
Is saying "digger drones" disrespectful?
Brad in Kansas City