How big are they now and how far down would you like to prune them?
they are between 24-36" tall. they're also getting a little too wide.
hand pruning now would be no problem. Try not to cut back into the brown stem part of the plant...junipers don't send out many new sprouts from brown stems. Try to cut only young growth.
and should i wait until september or october to do this?
It sounds like your plants are about the size they really want to be, but, as Walter indicates, it would be fine to do some minor pruning without waiting until fall.