We've hosted a number of couchsurfers, but have never stayed with anyone through that network. We have, however, stayed with LOTS of warmshowers hosts. We've found we typically hear back from everyone we send a warmshowers request to - occasionally they say no because they are traveling or whatever, but nearly always they welcome us to their home. that said, do try to write early - I try to write at least 2 weeks before we plan to arrive in an area. The few times we were turned down were when plans changed and we found ourselves in an area we hadn't planned on and contacted the warmshowers person to say, "We're here now - can we stay with you?" Whenever we've contacted them in advance it hasn't been a problem. One time, the warmshowers host was going to be out of town, so he arranged for us to stay with a friend of his!