I prefer to book accommodation (not, please, pre-book, the prefix is tautologous (yes I am an old pedant)) and plan my Camino as much as possible. (It's the only thing that gets me through the British winter!)
I know that many others prefer just to see where the day takes them but, like you, I prefer to stay in hotels/hostals, in which case booking is strongly advised especially at busy times. I have done my fair share of youth hostel/dormitory sleeping and discovered many years ago that the annoying person who snores all night and keeps everyone else awake is usually me.
I use
Gronze (in Spanish, but open it in Google Chrome and use the translate feature) and
Godesalco (in English) to plan stages and to see what accommodation is available. Many hotels can be booked through Booking.com, but bear in mind that you will pay a small premium for this over the hotel's usual tariff and that many hotels do not release their availability until February/March each year in which case Booking.com will misleadingly list them as unavailable for the date you want.
It is often worth contacting the hotel direct, especially if you can speak a little Spanish or at least send an e-mail with the help of Google translate. You will usually get a better price. I don't usually bother with breakfasting where I am staying, (Spanish breakfast isn't exactly a gourmet feast) but find a cafe half an hour down the trail.
If you're beginning at St Jean Pied de Port, you will certainly need to book if you plan to end your first day at Orisson. If, like most people, you go over the Route Napoleon in one day, you have the choice of either staying at one of the small hotels at Roncesvalles or walking on for a further 30 min to Burguete where there are two good hotels.
Good luck and good planning!