Take a look the POH or engine manual for the plane. Often you will see
oversquare settings and their corresponding fuel flow and percent
power. Obviously, if this is the case it is OK to run there. I actually
think that sticking to the published rpm/manifold is the best policy.
You KNOW that running the engine there is OK.

Note that if there are two or more settings of a certain percent power,
the setting with the lowest rpm gets the best fuel economy.

Some airplanes have a "redzone" on the tachometer indicating no
prolonged running the engine at that rpm.

Someone once posted that he called Lycoming to ask about vastly
oversquare running (like 1800 rpm and 25" mainifold) and if it was ok.
The engineer hemmed and hawed and said that Lycoming actually had never
tested the engine for long periods of time at those settings, but that
since it wasn't explicity restricted....well who knows? To me running
that low an rpm with that high a manifold pressure seems wrong. But I
can't actually proove it.

On a closing note, Lycoming recommends 1000 rpm immediately after
startup so you have enough oil pressure to cycle oil. Lycoming also
recommends no cooling faster than 50 degrees per minute (as I recall).
Best to stick with those guidelines also.