The first few days of a new class is when you decide your "roll" in the classroom. This is for university students who have a relatively small classroom size where participation is part of the class.
I would always assess the room to see who was going to be dominate in class discussions, who is going to sleep through it, etc. How you approach a class in the beginning is usually the tone you take for the rest of the semester.
If class participation is part of your grade or you know it will be necessary for you to establish a relationship with the professor or to help you understand the topic better, then make that decision from day one.
I always found it interesting that in one class I would be very outspoken and participatory while in others I just sat back and listened. Decide what is best for you from the get-go.