the double bases aren't going to hurt at all, we had a client come into our salon 100% gray and wanted her hair colored, called the tech hotline and the lightest we could go to still cover the gray would be a 9/03 which to me was very "yellow" and she decided against it,
I always use a neutral base because thats just what my clients happen to choose, but make sure that your clients percentage of gray hair matches the percentage of neutral colors in your formula
50% gray = 50% neutral in your formula.