If you just stick to the main train routes and/or the Wulumuqi vicinity, there is no need for a guide in Xinjiang at all (e.g. there are hostels at pretty much every mid-sized city on that part of the Silk Road that receive hoards of westerners). However, at most of these random cities, there are opportunities to go to some really exciting places that you couldn't access practicably in the absence of a guide (with an SUV and sometimes weapons to deter bandits).