For undercabinet lights, should you get the widest fixture that will fit, so the light will be even? For example, if I have a 30 inch cabinet, should I get the 28 inch fixture? (I'm looking at fluorescent lights: considering these T2 American Lighting Priori fixtures).

If I have a 15 inch inch upper, can I fit a 14 inch one, or am I safer with a 10 inch one so there's room for wiring? What about my 6 inch open shelves on the ends? do I just not bother lighting under those?

Here is a link that might be useful: Priori Fluorescents