Using local variables seems advisable in a partial that could be used application-wide to avoid dependencies across the application.
But within a single controller it seems acceptable to reference instance variables that you know will be available in all of the actions that use the partial.
If you do this, there seems to be a risk, however, that a particular action may get changed to no longer provide the instance variable to its view. Then the partial would stop working. I'm not sure if this is really a problem, though, since a regular view would encounter the same risk.
Does it matter if you reference instance variables in a partial?