Minor spoiler quotes will be accepted, major plot spoilers will not be accepted yet. These will be accepted at the date the DVD/Blu-ray is out in most of the world. This will be around mid-June.
If you're not sure what will be accepted, and what won't be, here is a quick list to see what would be accepted (scene names from the soundtrack):
These are just guidelines, things in these scenes might not be accepted yet, things outside of these scenes might actually be accepted already. Judge for yourself if it's too spoilery or not, and it never hurts to just submit!