There are many different stretcher bar profiles, and many different styles of cutting the wood. So it is impossible to say anything is "standard." There are also many big regional differences in the style and cutting of the wood, due to the historical reasons. For the same reasons, the wood used for making stretcher bars differs a lot from country to country depending on the forest that is present. But most stretchers, to avoid warping is made in well dried Nordic pinewood sourced from Scandinavia, Russia, and Canada.