As the other users already said: the most sites are using first the upvote (or like) button, and then the downvote.
Also when you say it. You say first upvote and then downvote.
With the voting system on Stackoverflow, you recognize firstly the upvote button, then the downvote.
Reanud's point with the forcing of the button is great. It is also good if you work with colors. Green for upvote, red for downvote.
On the other hand, spotify orders the buttons the other way:
First the downvote button, then the upvote. IMO, it is a really bad choice.
It happens very often to me, when I want to upvote a song which I like and just press and the very left button but accidently downvoted the track. Very bad is also, the downvoted track is then lost, so I can't listen it anymore.
If it would be colored, it would be more clear.
There are some examples where the downvote button comes first, but IMO, this is not a good use.
hjkl) and DDR put it! =P