When applying for postdoc positions, one has the option to send out lots of applications, possibly get several offers, then choose the one that seems best.
Alternatively, the applicant could contact the research group that looks the most relevant or best suited for him, and focus on getting a position there. If the feedback is not positive, then (and only then) move on to the next best choice.
I believe that in the USA it is accepted that people will shop around for positions, and not taking an offer is considered completely normal. However, in some other places it is possible that if the applicant shows a lot of interest for a certain position, then gets an offer to be hired, and doesn't accept it anyway, it could be considered rude.
What is the usual or best way for applying in various countries? I think it is very important to research this point to avoid any misunderstandings or bad feelings because of differences in attitudes towards the application process.