Yes and no. Usually a personal check is accepted for an initial (earnest money) deposit. However, some sellers may have specific requirements that a cashier's check only be submitted to escrow. If you are buying an REO, especially with a cash purchase, a cashier's check may be a requirement.

Any time you bring money in to close a sale (other than the initial deposit), a cashier's check or wire transfer is required. The escrow company will not take a personal check 3 days before closing.