Stretching paper isn't always necessary but I find it more enjoyable to paint on a flat surface. However wetting watercolour paper can wash off the size. I have a way to avoid this. Using Saunders Waterford paper 300g, I place it with the side I will not be painting on facing upwards.
I then wet this side only with a sponge being careful not to let water flow onto the other side. Then the paper is left for 10 minutes to soak. Excess water is paper towelled off and the paper is placed painting side up on the board and gum taped or stapled as required.
As the top surface is dry the tape sticks better and you can get masking/drawing/painting much quicker.