Well, it depends on your situation. If it's straightforward with no complications, and you are able to fill out a simple form, you don't need a lawyer. Otherwise, if you are on FBI most wanted list or incapable of filling out forms, then you may want consider hiring the lawyer.
Review N-400 and make sure you understand every single word in there. If you feel you have something to worry about (hide, not to disclose, choose the words carefully), then get a lawyer. Otherwise, you probably waste your money on the legal aid that you don't actually need. The examples of the interview can be found on USCIS web site, as well as YouTube. Check them out and make your decision.No just straight for applying us citizenship. Many of my friends had lawyer went with them for the interview. They did this way because they wanted to be safe in every step. But I feel the lawyer is very slow and not nice after you paid her or him. Right?