If we take a closer look at it, we might be able to see a difference. However, there really isn't any practical difference. Both sentences mean that you didn't want to open the gifts until they were gone.
till after they left - at any time that they are no longer present
till they left - immediately after - soon after - at the point that they left - and at any time that they are no longer present
So, once again, I say they both mean the same thing. There's just a small semantical difference, as I see it, that really doesn't matter.