Heck no, Best Buy warranties are never worth it, that is where they make their money. Also, AT already gives you a 2 year warranty:
From http://www.audio-technica.com/cms/site/9b1da7946d7de4bf/index.html#hp
U.S. Headphones Two-year Limited Warranty
Selected Audio-Technica headphones (models ATH-A700, ATH-AD700, ATH-CK10, ATH-CK1, ATH-CK52, ATH-CK6, ATH-CK7, ATH-CK9, ATH-CKM50, ATH-CM700Ti, ATH-EC700, ATH-EM7 GM, ATH-ES3, ATH-ES7, ATH-ESW9A, ATH-EW9, ATH-FC700, ATH-ON3, ATH-PRO5MSA, ATH-PRO5VA, ATH-PRO700 SV, ATH-SQ5ABK, ATH-W5000 and Import Series models only) purchased in the U.S.A. are warranted for two years from date of purchase by Audio-Technica U.S., Inc. (A.T.U.S.) to be free of defects in materials and workmanship. In event of such defect, product will be repaired promptly without charge or, at our option, replaced with a new product of equal or superior value if delivered to A.T.U.S. or an Authorized Service Center, prepaid, together with the sales slip or other proof of purchase date. This warranty excludes defects due to normal wear, abuse, shipping damage, or failure to use product in accordance with instructions. This warranty is void in the event of unauthorized repair or modification, or removal or defacing of the product labeling.
For U.S. service return instructions and procedure please go to:
www.audio-technica.com/returninstructions.
Except to the extent precluded by applicable state law, A.T.U.S. will have no liability for any consequential, incidental, or special damages; any warranty of merchantability or fitness for particular purpose expires when this warranty expires.
This warranty gives you specific legal rights, and you may have other rights which vary from state to state.
Outside the U.S.A., please contact your local dealer for warranty details.