Using on() or once() for `open` event in MongooseJS

I've seen a bunch of different ways of actually setting up a MongoDB connection:

1. I've seen some code where people don't use the open or error event
2. mongoose.connection.on('open', callback());
3. mongoose.connection.once('open', callback());

My take on it is:

• If my app only connected to the database when it needs to use it, use (2)
• If my app is constantly connected to the database ... it doesn't matter if I use (2) or (3)?

Which also raises the question, should my app maintain a persistent connection to the database (server and database running on same machine)?

Thanks for any help