When working on features, a teaser is usually graded before the main feature and trailer.
I find this a good way to start the creative process looking towards grading the feature film.
Touching the footage for the first time is always great, honouring the look references from the director and DOP, and giving them something they might not have thought about. Basically creating several looks that I feel could be right, just from my gut feeling. Having worked with the director and DOP on other projects always helps too. Knowing what they like / dislike.
A while after the teaser is done I start with main feature. The trailer is usually grading towards the end of the main feature grade, or after. I always ask for time to grade the trailer shots in context with eachother, to make smoother (or harder) transitions between sequences, depending on the project.
As Andy mentioned, a teaser / trailer is the "commercial" for the feature film.
I sometimes push the grade a bit harder for these short form pieces, since each shot has less time to capture the audience.
Shaping the light is also very important, directing the viewer to focus their attention on what is important for the story.
/ Dylan