I know people who say that the week before a marathon, they "starve" themselves of carbs for the first part of the week (the week of the marathon) and then carbo loading. Is that necessary? Is it helpful? Is it the only way to correctly do it?

I've heard of that, but I don't subscribe to it. I had a coach tell me once to make 50% of my calories be carbs while I'm training right up until the day of the race, and it's worked fine for me, so I haven't changed it.