There seems to be a confusion when asking questions of which of these tags to use. I have seen a few questions tagged with any combination of these.
Should there be some clearer guidance on when to use each of them?
Part of this confusion comes from the history of the various tags. When Stack Overflow launched, the OS for the iPhone was just called iPhone OS, and there was no iPad. Development for these devices was (and still is, in many places) referred to as "iPhone development." Thus, iphone became the de facto standard tag for all development questions regarding the iPhone and iPod touch.
Apple then rebranded the iPhone OS as iOS with iOS 4.0, which made more sense given the variety of devices other than the iPhone running that same OS. It has taken a while, but ios now seems to be the most popular tag for questions about development on these devices. I've actually made a habit now of adding the iOS tag to questions just tagged as iPhone or iPad when performing other edits.
While it could apply to most questions for iOS development, objective-c is less frequently used than those others, and usually only as a supplement to them. Same is true for cocoa-touch. We could argue about what's appropriate where, but this seems to be the convention that people have established on the site. Personally, I tend to use the former when a question explicitly involves language syntax or usage, and the latter when dealing with UIKit elements.
The really problematic tags are the version-specific ones that people use (iphone-sdk-xxx and so on), because questions tagged with just those aren't anywhere near as visible as ones tagged for just iOS and iPhone. I retag those whenever I can.
Also, don't even get me started on the misuse of [xcode] everywhere.
If there's any confusion it's because the user doesn't understand the respective topics enough. It could be argued that that user is lacking the prerequisite knowledge required to ask questions on those topics in the first place. I see no problem that an after-the-fact retagging couldn't fix.
If the wiki's aren't clear enough then a clarifying edit on your part would be very welcome.