If you are using GUI, then you shouldn't null variables. (The next time the trigger runs, that null value will be overwritten either way).
If you are using JASS, or custom scripts (containing local variables): yes, null them at the end.
Be aware that you must remove the location before nulling it as the null will throw away the handle for the object. You only need to null objects which have their handle index recycled (so local players do not need to be nulled) and only if you define the local inside the function (argument locals are not subject to the bug so do not need to be nulled, or so people say). You may also wish to null globals that will never be used again or are used very seldomly to allow the game engine maximum recycling efficiency.