Defragging?

You will get differing opinions
However, under normal conditions it is not necessary
It would be rare to need to defrag

 

The HFS+ filesystem is a bit "neater" and doesn't really need to be defragged*

My iMac is nearly 3 years old, my MacBook more than 4 years old, neither has been defragged (ever) and they both run fine.

* If you've used the computer for some time and you're looking to partition the drive without data loss, a defrag might be in order.

 

This recently active discussion has some great links in it.

This really isn't a matter of opinion—the technical details and requirements are rather clear. Most users do not need to defragment, pretty much ever. There are some limited or special circumstances in which you might, and one thing that could eventually bring it about is a full hard drive which is frequently subjected to the addition and removal of large files over time.