Safe to Delete Keynote Cache under /Library/Container/com.apple.iWork.Keynote/Data/Library/Caches
Cache is cache. Deleting it is safe. It's recommended to have all applications involved closed tho. Tools like Onyx are mostly the best way to just do a periodic cleansing every now and then.

If you just intend to delete that cache; no worries. Just close Keynote and remove the caches. You might want to remove the app states for that app too if this is a matter of something behaving weird in while working in it.