Is downthrust and right thrust always used?
some plane kits have it built in but if not should I add it?
Thinking 2 deg each.

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....

Agreed, the use of and the amount of down thrust depends on the airplane. Right thrust IMO is always a good idea but the amount depends on some variables. If you were to get specific on airplane/engine combo I would be able to suggest settings that would be a good starting point.

Thanks for replying guys...plane is Robin hood 99
99" wingspan....big high wing trainer type.
Engine is 40cc gas twin.
​​​​​ downthrust is built in but no right...

If I were building that airplane I would set the engine to 2 degrees down and 2 degrees right. The right thrust could be fine tuned according to prop size. Example, for a 40cc twin, a 18x8 prop will require a tad bit less right thrust then a 20x8 ( I think that twin will do either of those props ). Where you will see the biggest gain using right thrust on that airplane is less rudder workload on take off. While on the subject of less workload on takeoff, on gas engines I always extend the throttle arm so that I have a 2:1 ratio between throttle arm and servo arm. This allows you to use full servo travel and then adjust so that the carb opens up to 85% at full stick. That last 15% gives no additional power with these engines. The end results is a much more linear throttle curve and you can lean the high speed needle down just a touch more which will help eliminate the mid range burble common with gas engines.

On gas engines, I always use Amsoil at 50:1. Well, not always, I have found Redline to be good quality as well. I have a friend that uses Pennzoil with good results and others that use whatever is cheapest at the auto parts store, YMMV.

Astro