There are some posts
http://answers.unity3d.com/questions/3382/scale-the-performance-killer
http://answers.unity3d.com/questions/19595/meshrenderer-computescaledmesh
They say that if I scale a mesh, it is bad. Does this also apply if I scale the parent of the mesh, and only scale x, y and z by the same amount?
Scaling a mesh does cause it to take more performance than an unscaled version of the mesh, even if it is the parent (scaling a mesh through multiple parents is even worse). Unless you are scaling every single mesh in your game for some reason (without using import settings), I wouldn't worry too much about having a few meshes that are scaled dynamically. However, if they have mesh colliders, or even primitive colliders, you'll see even more issues as the physics system will have to calculate the new colliders as well...
Your best bet is just to scale things in the import whenever possible.