The "fork[1] if it changes things"[2] theory is one way of solving it, and by Occam's Razor it seems to me to be the simplest. But there are others, such as only closed loops being viable (i.e. the time traveller created the world from which they came because they came, like in Spock Saves The Wales where Scotty tells the secret of transparent Aluminum to the person who 'invented' it).

[1] "Fork it!" he said...

[2] I don't think it's needed for going to the future. Probably in our past no one has invented one yet, and when one is invented and someone goes forward that may cause comment then but we still don't know about it yet.