I see your point here, and in the wide expanse of American history that may be true, but over the last generation it has not been true with Republicans. Democrats always put people on the court with whom they agree - secular humanist leftists. Republicans tend to put liberals on the court that they were certain were conservatives when they made hte nomination. Bush has done fairly well, though he has given up on a few excellent nominees. But rememebr that the lobbying to keep Bush's appointments conservative has been intense behind the scenes.
I think it is far easier for the 'Rats to pick a secularist and and be CERTAIN that they will stay that way. Republicans have been fooled a bunch of times. However, the point still remains that no president ever has or ever will appoint a Supreme Court justice who they do not BELIEVE agrees with them.