You need to give Heptavac at least 4 weeks before lambing to ensure that the lamb get maximum passive immunity.
I don't drench mine - they have a mineral lick (Yellow Rockie) and a Crystalyx Standard bucket out plus they get a bit of cake from 6 weeks prior to lambing.
I do their feet, work and fluke them after lambing and just before I put them out on to clean grazing with their new lambs. I wouldn't turn heavily pregnant ewes to do their feet routinely. If one was lame, I'd check her feet in a standing position, like a horse.