Nope! eeros can be placed anywhere within range of each other to create a mesh network in your home. While a line of sight is optimal to reduce the chance of any interference between eeros, it is not a requirement.
Every home is unique and eeros are designed to work together through common barriers including walls, doors, and between floors.
Through the eero application, you can see the strength of the connection between each of your eeros and the gateway eero (the one connected directly to your modem). To read more, please visit this Help Center article: How can I find the connection strength and status of my eeros?