His reasoning may be wrong. The only reason I could think of hooking one up first is for quicker setup and this is really pushing the reason. Perhaps the DVR has a long boot up time. Or, he knows the DVR model is unreliable and as they do not carry extra he could have someone bring another while he continued working. The only other reason is a whole house DVR setup. The DVR issuing the addresses and again speeding up install instead of radomly plugging in boxes and maybe having to go back and reboot. Being SD boxes I doubt this an issue. You did not provide the make and model of the installer.