This may be an annoying phase in your tot’s development, but it’s an important one. She’s learning to decide for herself what she wants and doesn’t want (separate from what you, her parents, want). So, unless it’s a safety hazard, of course, encourage it.
If you really do need an answer, though, you may also want to experiment with questions that aren’t yes-no. In other words, instead of asking, "Do you want lunch?" ask, "For lunch, do you want cereal or a peanut butter sandwich?" If she’s saying no to something that isn’t really a choice — taking a bath, for example — stay with it: "I see that you’re sad because you want to stay downstairs, but it’s time to go upstairs to take a bath. Let’s pick out some toys that can go in the bath with you" or better yet, make a game of it: "Where are your favorite toys that go in the bath?"