As Sarah had warned Barnabas that she would not appear to him ever again unless he was good, I think she was keeping her word. However heroic we may have come to see Barnabas, however much we may consider him a tragic victim of circumstance, he continued to do bad, thoughtless, even cruel and evil things after the 1795 Flashback. Personally, I think she knows exactly who Barn is in 1840 1897 1995 etc. She doesn't appear because she is keeping her word (and she knows she is not part of these events).
Sarah is a curious creature. One is never quite sure exactly what she is all about. Is she simply an oblivious spirit who doesn't know she's dead? Is she fully aware of who and what she is and uses it? Is she there to stop Barnabas from being destroyed (obviously, the original idea was to destroy him). Her motives/agenda seem to change with her every appearance.
I would consider that Sarah, as a supernatural being, knows already much of what happened with Barnabas in the course of the series. Her appearances in 1967 were in part to try to save him. And I think save him she did. I think she knows/knew exactly what she was doing in appearing to help Maggie and Sam and Dr Woodard and David. In a way she was the catalyst for everything that happened after her mission was accomplished by spiriting Vicki into the past. With that one act, Sarah made the show we came to know come and love into being.
without Sarah's help, Barnabas may not have ever regain any kind of moral center - he would have been eventually killed, Willie probably killed, Hoffman certainly dead. Vicki probably would never have vanished into the past. The Adam storyline (sans his connection to Barnabas) would also have been bereft of Ang and Nicholas and Adam himself would probably have been Jeff Clark, though that would mean nothing to Vicki. Then of course, the whole Quentin Jennings affair would have ended in tears with both Jennings' boys dead (both probably werewolves that were destroyed, but then Quentin would have possessed David and Amy and they might have died, and the vengeful ghost would have succeeded in driving everyone out of Collinwood.
Not sure any of this makes any sense - I'm sick and cannot sleep and it's 2 am!