According to Leonard Lopez :
I would think it was a "never" rule, unless the flooring is _extremely_ stiff. Remember that any floor flex in this case will tend to open and close the inter-board spacing. Ie: cracking any finish. Allowing dust into the cracks. Ratcheting itself apart thru dust incursion. So even a tiny amount will amplify over time.
Is your floor "extremely stiff"? I have no idea what constitutes "extremely stiff" here.
Maybe the flooring manufacturer does.