I agree with Anders, but to answer your question: Using request admin means that Vista and higher (if UAC is enabled on those OSes) will automatically pop an admin request. That's all it does, nothing more, nothing less. But anything older than Vista (and anything with UAC turned off) will not do anything with the request, so you still have to check for admin access manually using the userinfo plugin. This is the proper way to do things: Either make an all-user installer at admin level to $PROGRAMFILES and HKLM (with both request admin and userinfo plugin verification), or make a single-user installer at userlevel to $(LOCAL)APPDATA and HKCU (with request user).
Like you said, using the UAC plugin will allow you to elevate at any point in the installation process. This allows you to circumvent the above choice, by doing things both at userlevel and adminlevel at the same time. This solution is however both very complex and quite error-prone. It is definitely not something you should be using unless you're A) overzealous, B) a masochist, or C) extremely sure of yourself. Or, preferably, D) all of the above.