Displaying numbers with the right sign
At least in older Numbers versions (in my example it's Numbers 3.2.2/Mavericks 10.9.5) you can define a custom data format.

Hit the "Cell" tab and the "Data Format" drop down menu > Create custom Format….

Use the default Number format and apply an additional rule like in the screenshot below:



and hit the OK button. Then apply the custom format Number+ to all appropriate fields.

The custom format is document based. You have to configure it anew for every new document.