While Google and Bing admit to using social signals for performance analysis, Google (at least) also admits that they do not look at Facebook likes and for good reason. It was a complete disaster when they did. Bing on the other hand did have an agreement with Facebook at one time and may still likely. The only real social signal Google may consider as absolutely trustworthy is Google+, the author tag, Google Circles, and so forth. In fact, the reason to use these from Google's standpoint has less to do with social media than with seeding their authority/trust database.
When you see the social buttons with values for likes, tweets, and so forth, this is not a value that Google looks at. But instead, the click-backs from social media is. For example, Google does not care if I tweet about or like your page, but they do care if you gain more traffic as a result. In this way Google is precisely correct. A like or tweet means nothing if it does not translate into new traffic in the end.
So to answer your question, do not social engineer your social buttons. Place them on all pages if you place them on any. You can never possibly know what is in the mind of your users and what has real value. 15-25% of all search are unique each day which tells you that user concerns change wildly. It also tells you that this is an unpredictable market space. It is far better to serve your user than not and not to chase what cannot be attained- perfect knowledge of what motivates a user. It is impossible. We can only know what motivates a user from history. Not using the social buttons, you will not only miss out on new trends/concerns, but will also be leading from behind.