Am I being evaluated at firm receptions, or are they simply meet-and-greets?
Yes, you are absolutely being evaluated at receptions. Until you have an offer, treat every interaction with the firm as an evaluation.
Receptions are a terrific opportunity to connect with more people at the firm, and to demonstrate that you would be a cultural “fit.” Dress professionally, keep one hand free for handshakes/introductions at all times (i.e., don’t have a drink in one hand and plate of food in the other), and create face time with as many of the attorneys as you can.
The guidance serves as a reminder that every interaction before receiving an offer is an opportunity to make a strong impression.