Throughout a number of Star Trek Series you see people going back in time to repair timelines. My question here is when they return to their own timeline would it still be left unchanged? As all they did by going back in time is create a new divergent timeline?
As has been noted, we don't know how time travel works in the Star Trek universe - and there seems to be some inconsistency depending on what the situation is.
In direct answer to your question, there is at least one instance of a repaired timeline holding at least some carry over - the DS9 episode Past Tense has Sisko and a couple of others cast back to a pivotal incident in Earth history. Unfortunately, their arrival causes the inadvertent death of the key person (Bell) in the event, which unravels the timeline. Sisko successfully intervenes in the role, and is able to restore the timeline - luckily, he paid attention in history class unlike Bashir, apparently.
However, when the crew are reviewing the historical archives of the event, they stumble upon Sisko's image - indicating some pollution of the timeline.