There are some risks with long term use of any medication including pred. She will likely need regular liver panels at least if she has to stay on the medication for life.

http://www.petplace.com/drug-library...one/page1.aspx
Here's a link with some pretty easy to read info on the use and risks of prednisone.

My Sleet used to take prednisone for allergies in the summer. She had such radical side effects that we found a different medication. Still a steriod, chemically very similar to prednisone, but Sleet processes it differently. It's not quite as effective or as quick in relieving her symptoms, but she has no negative reactions to it.

My Goldie is on pain control medication for life. It is quite likely that eventually the pills will damage her liver. She gets blood work every six months. Without the pills, she was a miserable creature who lived in isolation and was unpredictably aggressive. Goldie's life is likely to be shorter because of the pills. I'd rather have her live a shorter, happy, pain free life than a miserable long life.

You need to talk to your vet and decide if the risks are worth it. What's the reward for you and the dog in using the meds? Is her quality of life improved with the medication?