Unless you do some very specific tasks that are uncommon for normal users then you don’t have to worry about it. 

OS X has several built in technologies which automatically optimizes how files are saved to the hard disk in order to minimize fragmentation. While this won’t stop your hard drive from becoming fragmented over time it will slow down the process to the point where you might need to defragment once a year or so. Incidentally, installing system updates will force OS X optimize the operating system files and some commonly accessed files that are used during startup, etc. As long as you stay up to date you never really notice any slowdowns at all.

So when do you need to defragment? According to Apple you only really have to worry about it if you often work with video editing or similar tasks where you deal with very large files and combine this with only have very little free disk space. In this case you might need to defragment manually, but usually it gives a very small performance gain considering the time it takes to defragment the drive. A better solution might be to just move the big files to another disk for a few days and let OS X work its magic.

If you do have a drive you really think needs to be defragmented we’d actually recommend backing up and restoring your computer from a Time Machine backup instead of defragmenting it using a commercial tool. It gives you a good excuse to start backing up your files, it is generally much faster than defragmenting and it can usually solve other problems with your computer too. Not to mention that unlike the defragmentation tools available online, it is also free.