Saw and Solder (Applique) Champleve

I’ve been exploring saw and solder (also called applique) champleve. Some sources recommend using fine silver for the base and sterling for the top (design) piece. Others recommend sterling for both (and depletion gilding the bottom), while others say that for “small” pieces, you can use fine silver for both the base and the top piece. Can someone please explain to me why sterling is recommended at all?

And if you’re using sterling for the top, shouldn’t you depletion gild both the top and bottom or risk fire scale on the top piece?