Because it was built in the mid 1800s, the Palais Garnier, the Paris Opera House, has a different seating configuration than most contemporary theaters of today. The auditorium is arranged like a layer cake with levels stacked on top of each other.
Its enormous stage can fit more than 400 performers. However, the auditorium is small. The benefit of this smaller seating area is that just about every orchestra seat will provide a nice view.
You'll find that the orchestra section of the Paris Opera House is relatively flat. So for a ballet performance, if your seats are too close to the stage, it may be difficult to get a complete view of all of the dancers and the patterns that the choreography creates.
Also, while the boxes are the place to see and be seen, the box seats close to the stage on the sides may leave you twisting your neck and leaning forward to see the performance.
You'll want to get seats as close to the absolute center of the orchestra section as possible. The lower balcony in the center in the front rows is also good.
As for the cheaper tickets, the seats high up in the amphitheatre provide acceptable sightlines. After all, the auditorium is quite shallow. Still, the seats can be uncomfortable and the rows a little tight.
Finally, note that the even-numbered seats are all on one side of the opera house and the odd numbers are on the other. A seat numbered 6 will be next to seat 8. On the other side, seat 7 will be next to seat 9.