Title Case for Headings and Buttons
It's easier and faster for users if they can to identify the shapes of words.
"We recognize words from their word shape." also called the Bouma Shape.
Read more: http://en.wikipedia.org/wiki/Word_recognition
Bouma Shape: http://en.wikipedia.org/wiki/Word_recognition#Bouma_shape
A Few Examples
The English and Writers SE is Divided
This topic was already discussed in both the English and Writers SE . The consensus for the writers is it depends on the style guide set by the organization.
If non are established, writers may select one and are ok as long as they are consistent.
If there is a style guide your organization subscribes to, look in that. Otherwise, do what you think is right.
Examples of style guides are in one of the Answers:
It comes down to style and standards. It's more important, in my opinion, to be consistent.
My own theory is that internet users will always trend towards efficiency over grammatical correctness so I'm a strong proponent of Title Case. Why else has Urban Dictionary and Internet Acronyms come about?
My assumption is most users will spend less time on titles because they are deciding what content is right for them. Once they decide, the rest of the content follows sentence case and they can take their time if they need to.