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A scene is a small dramatic arc within a story. Usually scenes take place in a single place and time, though that is not always the case. Usually it is clear that one scene has ended and another has begun, thought that is not always the case either. Where there is a possibility that it may not be clear that one scene (one small dramatic arc) has ended, then inserting a scene break is a good idea. And if it is possible in one place, then it probably makes sense to insert scene breaks between all the scenes in the book. And since you are probably not the best judge of when it is obvious, you might be well advised to insert scene breaks between scenes all the time. 

Note that the difference between what you call hard and soft scene breaks is a typographical one, not a literary one. In a manuscript, you would be well advised to use asterisks and not rely on a blank line being noticed by the typesetter. After all, a blank line falling at the end of a page will not be seen at all.