Why stretch paper?
We always stretch our paper before attempting a painting (even heavier papers). Paper cockles so easily while wet, and it is all too easy to ruin a painting by having a mountain of paper develop across your work, complete with rivers of paint that refuses to stay where you put it and settles into stripey valleys of colour in all the wrong places. Paper that has not been stretched retains its cockled shape as it dries, and will not go flat again, so that even if you have carefully avoided the first set of problems you cannot frame the painting successfully.
If you would like to learn how to do this you may download this .pdf file: 'How to Stretch watercolour Paper'
If you don't already stretch paper, try it -your paintings are worth it!