Choices can't matter in ME3 considering you're supposed to be able to win no matter if you were full Paragon, full Renegade or something in between. That's why - judging from the leaked script - there's a McGuffin in the end to beat the Reapers.
By all means one would expect a Chaotic Stupid Renegade Shepard to be doomed to lose against the Reapers... making humanity stand alone, letting Illusive Man keep the Reaper base, destroying every faction that could be possible allies, etc.
Imagine the butthurt, though, if such a Shepard would actually lose. A modern game where it is possible to fail in the end, is very rare.