Arthur Daniels recommends you review any warning signs in execution plans:
Some things in life we ignore. For example, the “check engine” light. That’s just there as a suggestion, right?
But when you’re performance tuning, you can’t afford to ignore the warning signs. I can’t count the number of times that I’ve found the issue with a query by looking at the warnings.
The example Arthur uses involves implicit conversion, but there are several important warnings SSMS bubbles up.