It depends. If you expect to owe taxes, 2016 is already late, but you have an opportunity between now and April 17th, to prepare and submit your 2017 return on time and pay any taxes owed for 2017. However, if you don't expect to owe taxes it would be beneficial to work on your 2016 first so that 1. if you have a carryover to 2017 it would be part of your return; and 2. you can then import your 2016 information to your 2017 return and that will save you time.
