Maurice, unless you're going to park on steep sidings, I don't think your ever going to have a problem with them mixing. The level in the diff is lower than the axle tubes, and your probably packing your wheel bearings with high melting point grease that isn't going anywhere away from the bearings. The seals in reasonable condition will be enough of a physical barrier, which ever way you fit them. I myself, would however fit them to keep the oil out of the bearings.
You've probably already decided that anyhow.
