You probably will need to file, just to be safe. I know the attorney's fees were deducted from my back pay, being sent direct to the attorney. however, I still had to pay some copy fees not included in the amount the SSA paid him. The attorneys fees were tax deductible, so that helped on the taxes. Also, you can go back and file for years covered on the back pay so you are not claiming all of the back pay on this year. It will be broken down either on the letter you receive or the W-2 you get at year's end. I forget which one! I know I did not owe any taxes on my back pay, but as you know that depends on a lot of things and is unique to each taxpayer's status, etc.
I have noticed you seem to have a lot of questions and I wanted to let you know that the SSA website has publications online that answer most any question you may have regarding the disability process and what happens once you are approved. You may want to download and save a copy so you can reference when you think of something you may need to check. I know I found it very helpful.