The same stone, the same bed, the same bond. Where the quarry is worked out we find the nearest match and we put the substitution in writing.
Always. A joint is meant to fail before the block does, which is the whole argument against cement - it is harder than what it is protecting.
A weathered stone that is still doing its job stays. We have talked more clients out of full replacement than into it, and the building is the better for it.