When is best time to install sub-frame connector?

I don't think it matters.

If the car is together now, I'd put them under the car and jack them up against the floor pan, checking door gaps and ease of open close. Once you get it where you want it, weld them in. Not sure if that's a viable option with the US car tool style.

Cars have had them put on tires on the ground or loaded, under frame jackstands in varying locations, on rotisseries (sideways and upside down), doubt you could tell which was which when completed.

The manufacturing tolerances these cars is not good. When you have a car where the pass side wheelwell width measures 5/8" narrower than the drivers side, worrying about connectors and body loading isn't a big deal. JMO.