If your healer is consistently letting you dip below 40% and your are not dying, pat yourselves on the back; you're synced! And unless your healer is standing around eating popcorn, she/he is pumping out some serious dps.

My own comfort is keeping tanks between 50-90% and dps between 85% to full. I would never intentionally let a tank drop below 30 though, but it happens. That's why we have 'oh shit' heals in our toolkit.