Many hotels offering breakfast have regularly considered and even proceeded to splitting price lists between breakfast and room-only rates. It may work well on occasions, depending on how organized the kitchen is and how ready it is to feed guests deciding to book breakfast on the spot. However, if your breakfast quality is higher than average and your hotel is small to medium in terms of size, perhaps the best tactic would be to always include breakfast to your rates.
Excluding breakfast from rates and as all options are displayed before the bookings is made, the meal may seem expensive to a guest that would otherwise book the room with breakfast included anyway. Furthermore, even a guest that would not find breakfast an important service, could this way try it and eventually be overall more satisfied because of the breakfast quality. That particular guest would not experience that if Room-Only was an option.
The bottom line is: there is no one rule that a hotelier should follow; instead he should closely examine all facts for his specific, unique business before taking a decision like this.