Opinion: Both usages are allowed. It depends on what you really mean.

You can run a variety of scenarios to determine a variety of "cases". If you run scenarios to find a "case" that is worse than the one you're comparing to, then it is a worse-case scenario... you may be interested in finding "any" case that is worse than the nominal but not the absolute worst. If you run a scenario to specifically find the worst of all possible cases, then you run a worst-case. Remember, worse is relative to the case to which you are comparing and there can be many worse cases... worst is the most worse of all possible/known cases, thus it is treated as absolute since it is at the end of the spectrum of all possible cases.

The same arguements are true for best-case. If you run scenarios to find "better" cases, then you run better-case scenairios and to find the most-better you run a best-case scenario.