deadheading knockout roses

I have been deadheading this bush I bought last year by cutting under the hip, but read that you should cut above five leaves. The plant is looking great. I just bought three more at Home Depot and have two questions: can I continue to cut under the hip and should I fertilize-I don't believe I can get an accurate answer to find out if it was fertilized when I bought it. Do you think plants from Home Depot have been fertilized? I appreciate your responses to these questions. I have been gardening for seven years and still learning.

Thanks,
Karen

P.S. On this forum someone mentioned cutting below the five leaves.