Thanks Chris.
I'm trying to explain better.
I have write some debugs messages for the arrow item of Diagram scene.
I draw 8 'boxes' and its connecting arrows.

Ok, If I give the focus to QTcreator, clean the message window and give the focus to the diagram scene again I see the next :

bounding arrow 0
bounding arrow 0
paint arrow 0
bounding arrow 1
bounding arrow 1
paint arrow 1
bounding arrow 2
bounding arrow 2
paint arrow 2
bounding arrow 3
bounding arrow 3
paint arrow 3
bounding arrow 4
bounding arrow 4
paint arrow 4
bounding arrow 5
bounding arrow 5
paint arrow 5
bounding arrow 6
bounding arrow 6
paint arrow 6
bounding arrow 7
bounding arrow 7
paint arrow 7
bounding arrow 8
bounding arrow 8
paint arrow 8
bounding arrow 0
bounding arrow 0
paint arrow 0
bounding arrow 1
bounding arrow 1
paint arrow 1
bounding arrow 2
bounding arrow 2
paint arrow 2
bounding arrow 3
bounding arrow 3
paint arrow 3
bounding arrow 4
bounding arrow 4
paint arrow 4
bounding arrow 5
bounding arrow 5
paint arrow 5
bounding arrow 6
bounding arrow 6
paint arrow 6
bounding arrow 7
bounding arrow 7
paint arrow 7
bounding arrow 8
bounding arrow 8
paint arrow 8

'bounding arrow' is the message for 'bounding rect event' .
As you can see, there is two complete cycles (that is, the system paints things two times). And the bounding arrow is called two times before paint...

Another example of excesive bounding rect calls. First I draw 8 boxes and 8 arrows at left side of my view, set the zomm to 50% and draw two boxes at the right side. Ok, I see again how the bounding rect for the arrows from the left is called. I promise you that I have these first 8 elements at the left extreme of my 21" screen. There is no contact betwen graphic elements, so I dont understand these calls.

By now I only have seen setCacheMode () for backgrounds, or QStyleOptionGraphicsItem::exposedRect(), which works at item level. (Is there any equivalent for scene or view ?)
Which is the behavior for primitive items. In example, a QGraphicsPolygonItem() only needs the initial setPolygon() and setFlag's. I dont know if internally are a lot of bounding calls .

In short, what I'd want to know methods to avoid and/or help to Qgraphics framework does an unneccesary work.
Thanks