Do I need to stretch my paper?

There's no need to stretch your paper, as long as you use a glued block as recommended by Anna in her Equipment PDF. Using a block prevents your paper from wrinkling. 

Please see the video on Paper here in the Knowledge Base.

A glued block means the paper is glued around all 4 edges. This holds the paper down and prevents it from buckling when you're painting on it. The paint will dry before the paper has a chance to buckle. 

You can also minimise buckling by making sure your brush isn't too loaded with paint, dabbing it on the palette between picking up the paint and applying it to the paper. Using a smaller brush has a similar effect as smaller brushes can't carry as much water.

To rescue a buckled painting, you should be able to spray the back of it with water mist, put something absorbent directly onto it and then weigh it down with something heavy, e.g. a pile of books for several hours or until the painting is dry.