First, stretching the frame insufficiently to wind up at your desired width isn't helped by letting it sit for a couple of weeks. Metals don't "relax" so if it isn't the right width right away, it won't ever be.
Second, you don't need 135 mm dropouts to take an 8/9/10-speed rear wheel. Most 8+ speed road bikes use 130 mm hubs and getting a 130 mm hub into 126 mm dropouts by hand isn't that difficult. Stretching your frame a couple of mm, say to 128, would make a 130 mm wheel easier to install and shouldn't harm your frame at all.
Last edited by HillRider; 01-30-18 at 10:47 AM.