Hello Paul
It really depends on the platform you are using. Higher end platforms (6500/6800 with the appropriate supervisor as well as Nexus platforms for example) will support uRFP occurring in hardware thus providing for fast checking and no taxing of other resources.
If you specify an access list, you are essentially telling the router which range of addresses you want checked. So if a source is in the access list specified, the uRPF takes place. If it is not specified in the access list, then the uRPF check is bypassed.
I hope this has been helpful!
Laz