qt #include question

I just have a general question.

When including QT libraries, is it better to

a)

Code:

---

#include <QDomDocument>
#include <QDomElement>
#include <QDomText>
...More Includes...

---

or

b)

Code:

---

#include <QtXml>

---

I'm just wondering, at what point should you just use the overall generic include of the whole QtXml library, rather than each specific class/object that you use?

Is there speed benefits to using one or the other?

See thread: Qt modules