Do I need to defrag a Mac?

No.

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if you have a large number of files taking longer time to read/write, regularly creating a lot of files like photo editing files (making them smaller and then larger again in size).Fragmented Files can slow down your applications and longer system startup launch.

Support Link: http://support.apple.com/kb/HT1375

If your disks are almost full, and you often modify or create large files (such as editing video, but see the Tip below if you use iMovie and Mac OS X 10.3), there's a chance the disks could be fragmented. In this case, you might benefit from defragmentation, which can be performed with some third-party disk utilities.