Filler and paint will hide rotted wood for about a season. Then the filler cracks, the moisture that was already inside the board keeps working, and you pay twice: once for the paint job and again for the carpentry you should have done first.
You can check this yourself in ten minutes. Take a flathead screwdriver and walk the perimeter of the house. Press the tip, with moderate pressure, into the bottom edge of the fascia boards, the ends of the rake boards at the gable, the bottom of any door or window casing, and the lowest course of wood siding. Sound wood resists. Rotted wood takes the tip like firm cheese.
Pay particular attention to end grain and to anywhere two boards meet. Water gets into a cut end far faster than into a face, which is why the ends of fascia boards at a corner are usually the first thing to go. On the coast, from Huntington Beach down through the beach blocks, salt air accelerates all of it, and we routinely find soft boards on houses that were painted three years ago.
Also lean on your railings. Posts set into or against concrete trap water at the base, and a post that moves is a post that is rotted where you cannot see it. The same goes for deck boards near the house wall, where runoff concentrates.
When we estimate a job, we do this probe during the walkthrough, with you there, and the replacement goes on the quote as its own line with linear footage. That means two things. You find out what the repair costs before you commit, and nobody discovers it on day three and hands you a change order.
One more thing worth knowing: new replacement lumber should be primed on all six sides, including the cut ends, before it goes up. Most replacement boards that fail early fail because somebody installed raw wood, primed the three sides you can see, and left the cut ends drinking water for the next decade.