When is the best time to display a custom cursor for a tool such as a paint brush in a paint application?
One option is when the user clicks the tool and drags over the workspace, the cursor changes only over the workspace, when the tool is active.
Another option is to switch to the tool immediately when the user selects the tool providing feedback that the tool has been successfully selected, but not as much feedback as to where it can be used. Its a little odd too when then selecting other tools such as an eraser because you are selecting the eraser with the paint brush cursor.