If you revisit the code for maintenance there is some argument for using that as an opportunity to roll in strictures. But it's a bang for buck thing. With modern editors that perform background syntax checking it's pretty quick to identify where variables need to be declared and fix that. Because declaration gives a defined scope for variables it can also be a good way to think about how the code works. So the process of declaring variables can actually make the maintenance task easier.

If you have no other need to visit the code then generally it's best to leave it as is. Another reason to visit the code may be occasional flakiness. In that case adding strictures, especially warnings, is very compelling.