You either go site wide HTTPS, or should not bother with it in the first place. Should it be for all access or just for logged in users depends on the profile of your traffic. If most traffic will come from registered users, then just go unconditional HTTPS as doing it conditionaly just for logged in users obviously introduces more testing and might not play great with plugins that do not handle this situation well.
Side note: You should ask yourself why do it at all. Going HTTPS improves the general security against targeted attacks on specific users, but do not improve the general security of the site by much. It is more likely that a user’s account will be broken into by brute force then by someone “listening” to his traffic.