I think the dry lager yeasts really hit their stride in the 3rd, 4th and 5th use (the most I have taken one out is about 8 uses, IIRC - compared to 24+ pitches on a liquid lager yeast a while back). I religiously feed yeast nutrient to all yeasts that are re-pitched.
The first re-pitching of 34/70 can be a bit of a crap shoot, it seems, but I aerate the heck out of it (favoring O2 by stone with that situation) and give it a healthy dose of Wyeast nutrient in the last 10 minutes of the boil to give it the best chance to finish off well. I will overshoot Mr. Malty's suggested volume of yeast, as well on the second pitch of 34/70.
No science to back this, just anecdotal experience on this one.