I learned this lesson the hard way several years ago.
I can't stress enough the importance of making sure wherever you go isn't a place that just rubber-stamps W2 returns.
I made the mistake of going to H&R Block during the year I was 1099'ing, and aside from the person doing my return having to go ask someone for guidance at every line item on the forms, I ended up getting a letter in the mail from the state saying that they'd calculated I overpaid by a few grand.
I can't even fathom how bad of a job they must have done, if the state was the one to inform me that I'd given them too much money.