The BMS monitors the throughput of current to and from the battery, by connecting directly onto the battery negative terminal you would bypass the monitoring device and screw up the charging algorithms. By connecting directly to any other ground stud on the vehicle, you're ensuring that the charge and discharge current is seen by the BMS, that's all. Nothing bad will happen, just the vehicle systems won't operate necessarily as intended. Carrying out a battery discharge test through the BMS isn't a good idea though.