We put them under the tree about 5 or 6 hours before guests / family arrives. The main reason is the pets... they are way too curious. Before dogs, we would put them under as they were wrapped.
We have the opposite issue. I put the presents out as early as possible to keep the cat from getting into trouble, more specifically, the kind of trouble wherein she curls up under the tree and eats the (fake) pine needles.
