One advantage watercolour has over other mediums (and there are not too many comparative  benefits - only hardships!) is the ease by which a painting can be matted or cropped to create a better composition. 

The watercolourist should never be precious about the painting:

"I have many full sheet paintings that have ended up as 1/4 sheets or even as cards"

"With my (often too) complicated chosen subjects such as street scenes, festivals, marching bands the framed area to the painting is critical.  If I crop the chap in the yellow shirt on the far right the painting hangs together well.  I do not scrub out or change the original painting much - prefering to leave the painting fresh and evenly rendered.  The only thing I can resort to is the scissors."

The above is a painting of a Dublin Street scene.  Notice some matboard strips have been used to consider where the edge of the painting will be.

Some more sky was cropped for the final painting.

The painting is then matted to these dimensions and only then is the painting signed.