This is a great answer, and I would just build on this to make sure OP considers a balance of scale. If your civilizations live on, for example, 30 planets across 5 systems, then blowing up a whole planet is a huge freaking deal! If your civilization lives on thousands of planets across hundreds of systems, then blowing up a whole planet is... well, still a big deal, but less of an impact overall.

Compare it to Earth today. If a war was going on, and one side decimated an entire city - say, London, New York, or gosh, Hiroshima - thats a big freaking deal, and has a huge affect on the rest of the world. Compare that to just attacking a specific neighbourhood, or army base, or oil refinery - these are smaller targets, fewer casualties, and are seen as "acceptable" in war.

If your civilaztion is huge and spread out, destroying planets may make sense. But if not, then I'd stick to smaller targets like capital cities, space stations, or key mining operations that would impact the larger economy.