Are you referring to the control arms? If you are using stock control arms, with stock rubber bushings, you are supposed to put the car on the ground and then do the final torquing of the control arms with the vehicle weight on them. If you are running aftermarket control arms (and if you aren't, you should be with a tubular k-member and coilovers) then you can torque them down whenever, even if the car is still up in the air. Because the bushings aren't rubber, they aren't being preloaded (which is why it's a problem for the stock control arms and why they are to be torqued with the car on the ground).