We can set the default location in a purchase order which is considered as the preferred location.
This can be set by
Go to Go to Setup → Accounting → Accounting Preferences → Order Management
Under the Purchasing subtab, set the Default Location for Purchase orders

This can be done only for a single location only. We can choose any location over there. If the selected subsidiary corresponds to the location, then the location will be set by default otherwise the location field will be empty.
Then create a purchase order,

The same location is set by default. However, we can change it.
Note:
If we have to set default locations for each subsidiary, then we have to deploy a workflow for this purpose.