Spoilers are not necessary for answers. As far as I know, there has never been a site policy requiring spoiler-marked answers.
However, it is generally polite to spoiler answers if any of the following are true:
- The puzzle is short, so the answer will likely appear high up on the page.
- The answer has multiple parts that someone might want to reveal only some of. (For instance, it could be an answer to a logic puzzle that has many steps; or it could be for a puzzle with many "subpuzzles". It could also be an answer to a riddle or an optimization puzzle; someone stumped might want to know the answer, and then figure out the reasoning themselves.)
- The puzzle is being posed as a challenge (created by the asker), meant to be solvable even after an answer has been given. (Some people -- myself included -- enjoy doing cryptic crosswords that are shared here, sometimes days or even weeks later. Puzzles that are part of a series may interest people similarly.)
Most answers are spoiler-marked anyway; there's nothing disallowing it, and there are often good reasons to do so. (Ideally, all puzzles posed here should be solvable at any point in time; puzzles that are time-dependent are often not appropriate for the site. So any puzzle posed as a challenge should fit condition 3.)
But it's not strictly required. There's no rule against posting an answer without spoiler tags; you may specifically want an un-spoiler-marked answer, especially if spoiler tags are restricting your formatting for some reason.
In certain formats of question, there's sometimes no good reason to spoiler-tag: for instance, questions asking to identify a certain puzzle, or asking for the solution to a puzzle that not everyone can try (e.g. a mechanical puzzle). So, whenever those come up, I often leave spoiler tags off, and there's no harm in doing so.