Hi There - There are Pros and Cons to both methods.
If you make or just edit existing Styles for use with specific songs (you can adjust tempo and change accomp parts easily enough) then the big advantage here is that you can then use a MusicFinder database to quickly find the songs you want by either title, type or by some keyword you decide on.
However, I use registrations which allow for all of the above and some including loading the lyrics for a song and setting up the vocal harmony and mic sttings. I don't use SMFs (at the moment) but it would also load these as well if required.
I keep a seperate list of songs in different orders (Alphabetical / Genre / Artist / Tempo etc) and by each entry is the stored registration number - You can then call any registration by it's number even though it is not on the screen (Using the alternate list view for files).
Using registrations allows for many more variations of voices than OTS but I use it quite a lot to turn on and off or change the vocal harmony type but leaving the rest of the other settings the same.
The good thing is that you have the choice.
Regards - KF