Well, being that you grow only while at rest...esspecially off days since you are completely at rest, yes you need to keep the protein intake as high as training days. Optimal growth is acheived on rest days because you have not created a catabolic environment through training on that day. This sets up a prime oporitunity to recover and regenerate damaged muscle tissue...thus hypertrophy. I personally change nothing in my diet on rest days. It's the exact same as training days. Just because you're not training on a particular day does not mean you're not growing...quite the opposite actually.