Jay,
There are two types of SNC library available. Firstly, there are SNC libraries that include support for mutual user authentication, data integrity and encryption. These libraries are often used for SSO. These libraries are most often sold with a license (e.g. they are NOT free), and can be made available from SAP partners or from SAP (the SAP NetWeaver Single Sign-On product). Secondly, there is at least one SNC library that I am aware of which is free, but it only offers encryption, and no user authentication. This library is called SNC Client Encryption and available from SAP.
The first type of SNC library would require mapping (e.g. in SNC tab in su01), but would reduce the number of passwords that a user needs to remember. Depending on which SNC library you use, the capabilities to allow SSO to be turned off and whether domain trust is required will differ. You would need to ask the SNC library vendor to get more details of this, and compare.
When the second type of SNC library is used (as described above), there is no user mapping required (e.g. in SNC tab in su01) since the library is not performing any user authentication. This kind of library would require you to use a SAP password during the logon. I am not sure if the SAP Client Encryption library allows the user to logon and get an encrypted session without having domain trust. Maybe somebody from SAP can confirm if the client encryption library supports this, or if domain trust is required for this client encryption library.
Thanks
Tim