To expand on that point, I rather think it'd be a good idea to change the magic number in _combineHashValues at least once or twice every version.
This is based on the notion that a flexible design where the flexibility is never actually used becomes de facto ossified. Therefore, to prevent reliance on the precise formula for computing a synthesized hash, that formula should change on a regular basis as we reserve the right to do.
[cc @allevato]