Let's start off with a few questions to consider. Have you noticed any swelling, fevers, pain, drainage or other changes around the incision site? Have you had a recent tetanus shot? After the initial incident, did you thoroughly clean and wash your finger? Was the knife dirty or clean when this happened, (i.e. was the knife used for seafood, meat, or cutting any other food items like this)? These are important questions and the answers to these questions, along with your medical history, symptoms, and clinical exam will help your primary care doctor
decide upon the appropriate work up and treatment options that are available, depending on what is deemed medically necessary.
If your PCP recommended Ciprofloxacin, then this may be an appropriate decision, although many times (if the incision was clean and there are no signs of infection) you might be okay to continue to watch the incision for signs of infection - at which point antibiotic therapy may be more appropriate. Keflex is generally a good antibiotic for broad spectrum skin flora coverage, but again, the best first step is discussing this with your primary care physician.