What was the worm burden likely to be on the nursery paddock?
If low, no point worming lambs now. But you're right, they may well pick up worms when they go out with the adults.
Lots of folk will tell you not to worm, look at their bums, get worm counts....
All I say is that I worm lambs (and ewes) on pasture that's had sheep on, every 4 weeks for the first 3 months or so. Sometimes the only symptom of Nematodirus in lambs is death. The worms can build up very quickly if you get a warm, wet spell and lambs can quickly succumb. The NADIS forecasts help decide.
After mid summer, I start to go by mucky bums, bearing in mind that lush grass produces this too.
So - when did you last worm the ewes? What sort of worm burden is the pasture likely to have where they are?
If low then don't worm often. If likely to be higher, then do.