Since we have one vote for the primer/sealer and one vote that it is not necessary, I have to say that I agree with Steve. I think that a coat or 2 of B-I-N is really necessary. You stop any chance of the stain bleeding through and you make painting with a lighter color with the stain much easier. The "primers" for most paints are nothing more than a diluted version of the paint. A good primer/sealer like B-I-N will offer a much better gripping surface for the paint and the white pigment will block the old stain color. You can even get the B-I-N tinted but this will not be necessary in most cases.