I think it all depends.
On the one hand, any additional training you undertake, whether for a formal rating or qualification, or something informal like advanced handling or aerobatics, makes you a better pilot.
On the other hand, these additional ratings might enable (or even encourage) you to fly in more challenging aircraft and/or conditions which require more skill.
The safest pilot, IMHO, is one that is proficient, knows his limits and stays within them.