I would recommend the approach from Via delle Muratte, off Via del Corso, when you first go to see the Trevi Fountain. Merchants selling iconic memorabilia line the alley. As you walk closer to the end, you hear the sound of the water and the space then opens up.
Of course you want photos without all the people in the scene so you step up close and frame the shot.
At night, the hustle and bustle of people has disappeared and you can then step back and get something a little wider.
- How To Take Great Photos At Night