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- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
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Cross-posting is strongly discouraged: while you may think it's on-topic without modification on several different Stack Exchange sites, each site has its own culture and audience, and at the very least you should be tailoring your question to meet that.
To this point, there are very few questions that are on-topic on both Stack Overflow and Programmers.SE. Their scopes are purposely designed to be exclusive: Stack Overflow for concrete implementation questions and Programmers.SE for conceptual, whiteboard-y questions. And Webmasters.SE isn't even for questions targeted towards programmers at all.
So it's best not to even try to cross-post; instead, pick the site with the best fit. If you misjudge, the community can sort it out by migrating it to the correct site.
In addition to what Mark said, many users (especially across the sites you mention) participate on more than one site, so your question would have a high chance of being migrated to one location and its various incarnations merged together anyway.
You're generally better off picking the right/best place for your question and sticking with that.