It completely depends on the overall aerodynamics of the design. Some need it, some don't. I'd always initially build it as designed, unless there is specific real-world testing/advice to add some during the build.

Down thrust, in some planes, can be used to help limit the amount of pitch change with changes in airspeed. Not always, and not a cure-all. But sometimes it makes things better.
Right thrust, again in some planes, can help counter a yaw to the left, primarily seen on take-offs, or in tracking for vertical up lines. Ditto on not being a cure-all, though.

Do a search on the "Peter Goldsmith" method for trimming an aerobatic plane, there is some mention of when it might need a change in thrust. But that comes after building, and after lots of flight testing....