It’s all a matter of preference, of course, but here are a couple of related reasons why painting the stern is a good option. Most (though not all) A&A destroyer sculpts have transom sterns, meaning that they’re flat, which means that looking at the shape of the stern is already a good way to distinguish destroyers from cruiser sculpts (whose sterns tend to be pointy) – so by painting the stern, you’d be attracting attention to their most distinctive characteristic. Also note that if you paint just the flat vertical part of the stern, the paint won’t be visible if you’re viewing the sculpt from the front or the side. The benefit is that this doesn’t mess up the appearance of your sculpt too much; the drawback is that you won’t necessarily be able to tell at a glance if a sculpt is a destroyer, which may not be what you’re trying to achieve.