When in doubt, file an extension by 4/15.  It can't hurt and it avoids detonating a land mine you didn't realize you stepped on.

For clients where I know they get a K-1 in September or they like to wait and fund their SEP-IRA in October every year, I'm extending by 4/15.  It's not paranoia if they really are out to get you. 😉

The IRS has not exactly been crystal clear in official "published in the federal register" guidance and Tweets and FAQs do not constitute authority.