Sometimes. It depends entirely on the results of the restoration and might not be either required or desirable. If you have effectivly restored the original finish, there is no further treatment required. 95% of all antique furniture was originally finished in shellac. If you applied a coat of shellac, what you would be doing is adding a new finish, albeit the same substance as the original. Of course, any additional finish will add protection but you must realize that you will have a 'new' looking piece. Today, shellac is most often used as a base coat because of it's sealing properties.