Yes, if you know what you're doing (edit: and everyone else with access to it does, too...), you can ignore this warning.
It exists because even large organizations who should know better have accidentally placed private data into public buckets. Amazon will also send you heads-up emails if you leave buckets public in addition to the in-console warnings.
Accenture, Verizon, Viacom, Illinois voter information and military information has all been found inadvertently left open to everyone online due to IT bods misconfiguring their S3 silos.
If you are absolutely, 100% certain that everything in the bucket should be public and that no one's going to accidentally put private data in it - a static HTML site's a good example - then by all means, leave it public.