Stretching your watercolor paper has certain aesthetic advantages. You'll end up with a perfectly flat surface on which to work. This surface will stay relatively flat as you work and the finished painting will dry perfectly flat. The only disadvantage is that it takes some effort and dedication to do it consistently. Any paper of less than #300 weight is bound to warp according to the amount of water absorbed in the painting process. Synthetic papers generally do not warp at all. Watercolor paper in prepared blocks will dry flat also, if left on the block to dry I use 140 lb paper for the paintings that I sell on ebay so I almost always stretch it. I find 90 lb. paper is too flimsy for serious watercolor work. When I use 300 lb. paper it doesn't require stretching, but it is expensive so I use the 140 most of the time but love to paint on 300 lb when I am really working on a large painting I use 300 or 200 lb paper. When Stretching watercolor paper I allow 1” for g...