(Most of this should be elaborated on at some point.)
They’re fine as long as they’re at and won’t be animated.
Triangulating
Don’t triangulate your models.
Inverted normals
By inverting the normals of a model you can achieve an interesting pseudo transparency effect.
It’s not realistic but we’re not aiming for realism.
Double sided
Many thin objects such as paper, leaves or cloth don’t need to have proper thickness, but for performance the engine does backface culling, where faces that are “backwards” aren’t rendered.
This can be avoided by selecting the mesh, duplicating and then inverting the normals.
Pivot
Always at the center (XZ) and base (Y) of the object.
Modeling with NURBS
Cables and wires, but also some round but geometric objects like the plastic “cup” chair.
Cell fracture
To shatter objects for ingame destruction. Takes some setup.