Off Broadway, Off Off Broadway, workshops, and other similar producing groups have long since become indispensable to the musical comedy scene. Some shows graduate to the commercial big time. In fact, most of Broadway's biggest current musical hits - including ''A Chorus Line,'' ''Annie,'' ''The Pirates of Penzance ,'' ''Dreamgirls,'' and ''Nine'' - originated outside the commercial mainstream. The same could be said for ''Pump Boys and Dinettes'' and ''Joseph and the Amazing Technicolor Dreamcoat.''
Yet for certain entertainments, the modest and sometimes even scruffy milieu of Off Off and Off Broadway seems the natural environment. ''The Fantasticks,'' grand ancestor of them all, has been running since 1960 at the Sullivan Street Playhouse. By now, it should merit landmark designation. ''One Mo' Time'' is in its second year at the Village Gate.
Among more recent arrivals have been ''Little Shop of Horrors,'' ''Charlotte Sweet,'' and ''Lennon.'' The last mentioned, a stage biography of John Lennon that premiered last season in Liverpool, is now previewing at the Entermedia Theater. ''R.S.V.P.,'' a musical review about New York, opens this week at Theater East. It is directed by Word Baker, who staged ''The Fantasticks'' as well as many other productions.
The other two late additions to the Off Broadway musical roster both transferred from Off Off Broadway. ''Little Shop of Horrors'' first saluted the new horticulture at the WPA Theater. The Victorian heroine of ''Charlotte Sweet'' began her adventures and misadventures at the American Theater of Actors. Both shows are in the tradition of latter-day camp.