To be honest it'll depend on how you are using it and for what besides Rosetta.
If it's on just a few hours a day/week and used for other stuff simultaneously, then no, it won't be worth it, as you won't finish a single project in like forever
If however, it's just left on and crunching on a shelf somewhere, then yeah, you'd not see huge number obviously, but it'll contribute.
One thing to bear in mind, is how much power it's actually using while crunching, even if the netbook itself won't draw much power, some will be lost through the power adapter as heat.
In a worst case scenario, you might look at a minuscule contribution for quite a lot power.
However bearing that in mind, Rosetta is actually what I chose to throw on my laptop and had running awhile back on my netbook, simply because it's a lot more forgiving on time limits and project sizes than comparable distributed computing, like Folding at home.
And no, I can't actually remember precise numbers for the netbook and I was also using it for other stuff (think big demands of a small hardware package; apache, virtual machines, 30+ chrome windows, and a ton of other stuff normally reserved for desktops by any sane people).