There aren't any reliable statistics that show whether a subject line that never varies will draw better or worse than one which changes to reflect the new content in each issue. You can test this on your own audience, however, using a standard A/B split test.
Divide your database in two. Email the newsletter with the standard subject line to one segment, then create a unique subject line for the other segment. Do this for at least three publishing cycles, and always send the same subject-line configuration to the same segment of your database. Then, compare the results. If you see a difference of more than 5 to 10 percentage points in your open statistics, it's probably significant enough to drive your decision.
Why Go Unique?
When you repeat the subject line over and over, you don't give readers a compelling reason to open the email. We believe the best practice is to create a unique and compelling subject line for each issue.
You can create a subject line that's both instantly familiar AND unique each time by incorporating your newsletter name in either the sender line (the "from" line) or by putting it first in the subject line.