It really depend on the area where you drive. Sometime you can do it with a couple of point in 100Km but sometimes you need to use as many points as Marius is saying.
I'm making a small tour for tomorrow. (150Km) from my town near Amersfoort toward Nijmegen and back.
In 1 stretch of my route from km 30 to km 60 I use 8 points (so every 3.7Km) just to make sure i ride the route, that I want to ride. Because if I don't do that the route will change.
So for your questions;
I would place a couple of shaping points. Just to make sure the route is to your wishes.
But in a area with not that many roads or alternatives you don't have to place them every 3 or 4 km.
In my last road trip trough South-west US there where days with a couple of shaping points for 250/350Km and most of them where highlights/viewing points and not needed for shaping the route.
So all-in-all with my basics experience, it really depends on your are and your own wishes about your route.