I've read before that it is advised to install frame connectors with the car either on a 4 post-drive on type lift or on jackstands with the weight of the car bearing on its tires.
The big thing is to check the doors for opening and closing before you weld, if they function as they should and the gaps look good up on the lift then you'll probably be fine.
I would check them before lifting, and after you raise it to see if there are any appreciable differences.