As explored in another SCF question, the only way to answer the bootstrap paradox is to say "every time someone is sent back in time, time branches into a separate line."
But if that is the case, what is the point of Skynet sending anything back in time? The chances of the same "Skynet" being created is virtually zero, so why bother?
And if we just say "It just wanted to win the war and let another machine exist in another timeline," what is the point of sending Kyle Reese back in time? Humans were winning already, let the machines send a Terminator back in time, who cares? The changes it can do will not affect the current timeline they are in. And it just doesn't make sense for them to care about that "other timeline", because if there is one other timeline, there must be infinite number of timelines.
Edit: Ok guys, I don't think there "must be" infinite number of timelines, you are right, that was poorly thought and worded. My thought was:
If time travel is possible and invented once, it is likely that it will be invented again, and each time it is invented and used, there will be a separate timeline. And we might think that it is unlikely that any given timeline is the first timeline with time travel, if we consider it is likely that there are many. Given infinite amount (or very long amounts) of time, I would think this would happen enough times to create a big number of separate timelines.
This is not a vigorously thought scientific hypothesis, it's just how I thought of it during the timeframe where I was watching the movies, so don't expect much from it :)