There are no hard and fast rules regarding whether cabinet knobs have to match hinges. The decision of whether to match them or not is largely based on personal preference and the overall design aesthetic you're trying to achieve.
Matching cabinet knobs and hinges can create a cohesive and coordinated look in your cabinetry. It can provide a sense of unity and consistency, especially if you prefer a more traditional or classic style. For example, if you have brass hinges, you might choose brass knobs to complement them.
However, mixing and contrasting hardware finishes can also be a deliberate design choice. It can add visual interest and create a more eclectic or contemporary look. For instance, you might pair black hinges with brushed nickel knobs for a contrasting effect.
Ultimately, the decision depends on your personal style, the overall design scheme of the space, and the specific look you want to achieve. It can be helpful to gather samples or images of different combinations to see how they work together before making a final decision.