If you made under $100,000 from salary and don't otherwise have any complications (like dependents), a form 1040EZ really is simple. There's no reason to use software for that. It's quite straightforward.

If you have deductions, stock sales, a nanny, a business, etc then you need the regular 1040 and various schedules, and those are all complex enough that you'd probably benefit from software. It's not absolutely required, but there are enough ways to do it wrong (like adding up the wrong lines) that the peace of mind alone is probably worth it to you.