If the place is popular, you could be disappointed if you arrive without a reservation. There are more and more restaurants in Paris that use tripadvisor for reservation booking, which is simpler than calling or emailing to ask for reservations. If you don't have reservations and the place is popular, it's best to show up when they open to get a table.
What time are you planning on eating? If it's early, you won't have problems eating at the places that are open all day, which tend to not be as good 'service continu/continuous service'.
There are a lot of very good restaurants in Paris but it depends on what you want. One of my favorite restaurants is Restaurant de la Tour which is close to the Eiffel Tower. It's run by a husband and wife team. We live here and go a few times a year. Although it is in a tourist area and caters to tourists in the evening (when you need a reservation - reserve by email), it caters to a local lunch crowd and at lunch you generally do not need a reservation. Look it up on tripadvisor for reviews and pictures.