SOme spouts have a gasket. IF yours does, it shouldn't be a problem. IF not, then it's not a bad idea to caulk it. Does the spout screw on or is it a push-on type? If it screws on, you might be able to get another turn on it. IF it's a push on, if there's any movement in th pipe, without pushing against the back while pushing it on, you may never get it tight. Normally, the pipe behind would be anchored. Does the spout pull out if you pull on it or is it solidly anchored?