I don't have an answer for your question. However, I might have an idea of why this might be relevant. It might be relevant in case of bigamy, or of people who got visas as unmarried sons or daughters, when they were actually married. The government might use this information to ask their embassies or consulates abroad to obtain information about those previous marriages. All said I don't have any idea if this is done regularly. Couple of thoughts:
1. If everything is legit about those marriages, it is possible to get away by just putting year and month, but be prepared to furnish actual dates during the interview if the officer asks for this. The officer might also ask for copies of the marriage certificates at the interview if they want to. In particular if this has never been provided before as part of some other immigration benefit (like the Green Card) It all depends on the officer.
2. If the dates given are grossly inaccurate, and intended to hide some prior immigration missteps then all bets are off. It is possible that they don't look further into it, but it is anybody's guess, and I wouldn't suggest anybody to try this.
If USCIS already has accurate information about these prior marriages (your husband submitted copies of marriage certificates during Green Card or other filing) I think putting year/month should be fairly safe as they can always go back to the record to verify exact dates. Anyway, best advice is to fill out the N-400 as completely and accurately as possible, mistakes can always be made, we are all human.
