You should find that a fair few planes have provision for dispatch with the gear locked down, usually for a ferry to a maintenance base in the event of a problem at an outstation. The flight manual should provide the guidance in such cases.
Looking at one such supplement for a small, twin-engined a/c, I see a WAT penalty of about 15% (so you look up your WAT limited weight and then have to knock off 15%) and also climb gradient penalties of from 2.4% to 3.3%. I'd expect the penalties to be highest for a small a/c, since the gear probably is a larger component of drag in such cases (but of course, not all).
I'd interpret that as roughly that any TO within 15% or so of your limiting weight is a potential issue, depending on the actual obstacles.