"I am not a punctual person."
Is "一个" strictly necessary here? If not, is there a difference in nuance with it versus without?
In oral Chinese, they almost certainly would leave out the 一 (我不是个准时的) but I think for the most part people are going to use the measure word.
That sounds right. I wonder if leaving it out would make it mean something like "the punctual person", e.g. the punctual one in the group.
Well, I'm not a native speaker so I may be wading way outside of my expertise, but I have lived in China for a long time and my oral Chinese is pretty fluent. I think the distinction would be similar to, "I am punctual" versus "I am a punctual person". For instance: 我是中国的 (I am Chinese) vs 我是个中国人 (I am a Chinese person).