I have a love-hate relationship with most lilacs!! but the idea is to deadhead them too when they are finished blooming. Depends on how much of a neat-freak you are, though, because they can be left -- the point of deadheading is to try and get something to bloom again later in the year. Lilacs will not, in my experience. Usually I deadhead perennials and am rewarded with extra flowers, as well as a neat appearance.