To be honest, this is why I don't believe the line of demarcation is as weak as people make it out to be. There is ALWAYS a line of demarcation on your head because your new growth is always growing in, but your hair isn't always breaking off.
It really depends on what you prefer. Sometimes I part my hair in sections and moisturize the length, but when I'm wearing my hair straight, I don't want to weigh it down with product so I just rub the moisturizer over my hair and on my ends. Water/spray moisturizers work well for textured styles and buns, but in my experience butter and oil-based moisturizers work better on straight or rollerset hair. If your deep conditioning is done well, you might not have to worry as much about moisturizing the whole length of your hair. But, definitely don't neglect your ends!
During a long stretch, I apply a creamy leave-in with really good slip to my new growth to help with ease of parting and styling after I wash and condition my hair. I relax every 6 months.