This could be problematic to use the MAC address...it can be changed, and may be reported differently, particularly on laptops, since the laptop has probably a wireless and wired MAC address. The Agent will report back all MAC addresses for each NIC on the machine.
If possible it is preferable to use the system serial number for Asset management (unless you're dealing with a lot of "white box" PCs vs. HP/Dell/Lenovo-branded PCs).