Well for this exercise, that's actually the point. Since they're triplets, you start with a down stroke, but then the next triplet you're going to be playing the open string with an upstroke. So for triplets it doesn't really matter what you start with because you're going to be doing both down and up strokes on the open string.

Now, for playing licks with an even number of notes on each string, I would start with a down stroke (I think most people would). Although I do force myself to practice chromatics starting with an upstroke just to get used to it. As John Petrucci said, you have to be "ambi-picktrous."

Here's what I used to create that tab: http://www.guitartabcreator.com/