That's why I've never delved in time-travel: Too many points of impracticality to overcome, as well as the fact that the lack of evidence of time-travel in our past indicates no one's ever figured out how to do it!
Seriously, if I was going to subscribe to any theory, it would be the linear time theory, coupled with the fact that a time traveler would automatically become part of the timestream when he arrived, not a time-disrupter...ie, whatever they may do has already been determined by history, they cannot change it.
But then there's Star Trek...
Bottom line (and to be considered a rule):
Time travel is just as impossible as FTL drives. If you can ignore the science enough to have one... go ahead and do the other... what's the difference?