Is the 1968 Roosevelt dime without the s mark rare?

No. The US Mint produced over nine hundred million 1968 Roosevelt Dimes without an "S" mint mark.

Unless it is truly a Proof dime from San Fransisco. It would be more than likely in a proof, unopened set. Apparently, they produced the proof coin (which started in 1968 in San Fran) missing the mint stamp. They caught it but some got out I guess. I think that is where the confusion stems from. This coin didn't have a mint mark from one of the other mints.