Just let them finish...bud with a few seeds is better than early bud with a few immature seeds...may as well wait til the seeds are viable...that way you get good smoke and some seeds...
When people say that seeds take down the potency and yield, it is because if you totally seed a plant there is nothing left to smoke, but the little pods the seeds are attached to the plant in...but a few seeds here and there doesn't take away from anything...chopping early will take away more THC then the finished seeds would.