Do I need a rental car for my trip to Seattle?

That depends. If you are staying downtown and have no plans of traveling outside the city then I would say no, you don't need to rent a car. There are plenty of transportation options at Sea/Tac Airport as well as downtown. Once downtown, you can walk or take a taxi to most areas of interest. However, if you are planning on making any short trips (which I highly recommend), you really do need a car. Many great destinations are just a short drive from the city. I've had my best experiences renting from Enterprise and Budget at the airport location (if you are looking for a recommendation). All car rental desks are next to baggage claim and the rental car shuttles are across the skyway next to the parking area.