You should hire someone to do your taxes if . . .
. . . you can afford it.
. . . your return is more complicated (multiple rental income, complex business expenses, etc.). If you have lots of questions about what items you should include or cannot include on your return, it may be helpful to pay someone to help. Also, professional preparers can sometimes find tax credits or deductions that you wouldn't even be aware of.
. . . taxes totally squick you out and you're not sure if you would do it correctly. If paying a professional will give you peace of mind (and you can afford it), it may be worth it.