Time Intelligence with Monthly Data Model

Sounds like the right approach. 

Agree, I routinely create models with monthly transactional data, and simply follow a first-of-the-month or last-of-the-month date convention with dates in the fact table (can't see any reason to prefer one over the other), related to a standard daily calendar table. 

As long as reports don't filter below the month level, it works just fine with time intelligence functions.

There are methods to allocate monthly data to days (some ideas e.g. here) but there is no point in doing this if your underlying granularity is monthly.