There really Isn't any Should or Shouldn't when it comes to what Wattage to use. Because so much depends on the Type of e-Liquid one uses, Amount of Airflow, and of course what type of Hit one Likes.
As a General Rule, lower wattages will make a Coil Last Longer. But I think someone should just use what they Like the Best.
If 40 ~ 45 Watts is what works Best for you and your e-Liquids, that is what I would suggest you use. I might like 20 Watts with that same Set-Up. And the Next Person may like it best at 60 Watts.