It bears repeating that no-clean fluxes are only guaranteed to be safe for their stated purposes once heated fully to reflow temperature. When hand-soldering using only the flux in flux-core solder wire, the flux is guaranteed to reach that temperature. But when hand-soldering with external flux (paste, gel, liquid), it may not get heated at all, or may get only partly heated. Unheated or partially-heated flux may be left in an active state and be corrosive. In those cases, flux residues must be removed.
The same applies to many rosin fluxes, which may be corrosive if still wet or pasty. (In tests I’ve done, some liquid fluxes caused no corrosion at all when applied in a thin layer that could dry within a short amount of time, but droplets big enough to take a day to dry caused corrosion.)
My approach is this: when hand-soldering using no external flux, or when using solder paste on a board then reflowed in an oven, I consider cleaning as completely optional. But as soon as liquid or paste flux is used for rework, cleaning becomes mandatory.