Use of the Peer class in lib/model/
You can use this if you need to get any datasets from the database
public static function ....
Use class in lib/model/
Put code here when you need to call functions from an object, even if that is linking to another object.
Use of Criteria
When using SQL statements, don't worry about converting dates and strings to work in SQL, Propel does it for you.
Clear cache
Clear cache when you think you need it, it does not matter when you are developing. Better to clear cache and get the latest changes than not.