The 5 mg might be just right.
It is my understanding that if you have been on steroids just before surgery - and in some cases, within six months or a year of being on steroids, then it might be necessary to be on steroids during the surgery.
Why? Because the body's ability to deal with physical stress can be compromised even after a brief round of steroids, and surgery is certainly physically stressful.
The following link leads to more info about prednisone, including the impact of surgery:
http://74.125.47.132/search?q=cache:5s5EQK-2hUAJ:www.gihealth.com/html/education/drugs/prednisone.html+prednisone+surgery&cd=1&hl=en&ct=clnk&gl=us