Based on my own experience, even if your last name comes before your advisor's, your own research community will regard any joint work with your advisor to be primarily your advisor's work, despite your advisor's protests to the contrary, until you start publishing independently.  The [Matthew Effect](http://en.wikipedia.org/wiki/Matthew_effect) is a _much_ more significant than your position in the alphabet.

(My name comes before my advisor's, and I've advised students with names before mine and others after mine. I work in a field that orders authors alphabetically.)