Vent risers are only needed where there is not a constant fall in the pipeline. Even a small slope is enough to stop air bubbles becoming an air pocket. Yet even a small high point in the line will usually form an air pocket and so reduce the flow. The temperature and velocity of the flow does have effect on the likelihood of air pockets/locks forming, but the safest design is to have some fall all the way.
Rgds, Nat.