Strictly speaking, writing something negative would be biased the other way, while writing nothing is neutral. But I see your point that not allowing the fact o

Strictly speaking, writing something negative would be biased the other way, while writing nothing is neutral. But I see your point that not allowing the fact of it to change your behavior at all is the perfectly ethical solution. It is also impossible for humans to accomplish (and moreover to even know for certain they've accomplished it), so it's a moot point. There is also something to be said for avoiding even the appearance of impropriety, which yours doesn't address.