That is only if you have low fertility. In wyandottes, the rose comb gene, if it hasnt had any new blood introduced, can hurt the fertility. You dont really have to worry about that here in the U.S. because there have been so many lines of wyandottes poping up with single combs so we dont really have to worry about it. In show lines, you might worry about it, like in our LF silver laced we had a low fertility problem and never got any single combed chicks out of the line, but when we sold them to another breeder that combined ours with theirs, the fertility was great because of almost hybrid vigor. Neither lines had had any new blood introduced in ages, so by breeding to fresh blood it gave the line a 'fresh start', the pullets from the combined lines were great layers and the new owner doesnt have that many fertility problems at all. So to summarize, dont worry about crossing in a single combed bird unless your fertility is horrible and you havent tried a new line of the same variety.