depends on the style cabinets you have.

Ours are at the back. I don’t know if there is a “rule”.

Mine are at the back. I have GE ones, and the light sits about 1.5" or so off the back of the cabinet. The light fixture itself sits flush against the back of the cabinet, but there's about 1.5" that is metal, so the light itself is not right up against the back. The light has an angled profile, so it illuminates the work area of the countertop just fine.