Poll results:

Null return type instead of void:
+2 jakub, richard: true return type
-3 tony, derick, friedhelm: isn't so explicit as void
+0 etienne: differentiate between really void in echo and null in other functions
-3 dave, hartmut, friedhelm, nuno: add void to Pseudo-types instead

Remove void from empty parameters list:
+1 jakub: same as user-defined functions
-1 hartmut: isn't so explicit as void

Thank you for the opinions, I've added void to Pseudo-types.

Jakub Vrana