If you can, leave the name blank. "127.0.0.1" should work, but it goes different ways internally, both inside QuickOPC, and inside Microsoft COM/DCOM.
Basically, whenever the machine name is not empty, it is (for most part) "unrecognizable" from representing a remote computer, and it is treated as such. It may mean less efficient access, and small differences in functionality.
One of the things that differ is that with an empty machine name, CoCreateInstance API call is used to instantiate the OPC objects; with a non-empty name, CoCreateInstanceEx is used, and this one takes many more parameters, regarding security etc. This stuff can hardly bring anything useful locally (and in some cases, it may even cause issues).