Ignoring the actual FTL aspect of the question:
In the comics, Doctor Strange has been known to travel in time in multiple stories. In particular:
In Doctor Strange #17, August 1976, Doctor Strange and Clea travel through time to celebrate the United States' bicentennial:
In Doctor Strange #48, October 1981 (and continuing at least through issue #53), Doctor Strange travelled into the past, crossing paths with Sgt. Fury and his Howling Commandos during World War II (Note that Doctor Strange and S.H.I.E.L.D. shared Strange Tales for a few years in the mid-1960s, and that this story ran through issues 50 and 51, thus covering an anniversary issue), and (later) the Fantastic Four and Pharaoh Rama-Tut (crossing over with Fantastic Four #19, published years earlier).
Neither of these stories goes into much detail about the possibility of damaging the timeline. As I recall, the impression we are given follows the school of thought that, if you travel in the past, whatever you did is already a part of your history, and thus won't change the past.
That said, there's nothing preventing someone from writing a story where his time travel puts the timeline at risk - just that, as best I can recall, such a story hasn't been written yet. It makes sense, really - that's a really solid SF concept, and Strange is the Sorceror Supreme, not the Scientist Supreme.
OK, to touch on the FTL bit a bit: Strange's ability to move from one place to another has generally been portrayed as something that bypasses the distance (in SF terms, like a wormhole), rather than something that transverses the distance faster than light.