I wouldn't say its bad. However a few degrees for us is a bigger deal for a snake. If your tank changes temp a bit that's fine as long as its within their good range. I would make sure to have a constant basking area though. Its easy for your snake to move to a cooler zone but if it doesn't have a basking zone when it needs it, it can't create it for itself. Try to make sure to keep that constant. I Use a ceramic heater on a stand. That way I can lower or raise it to get the desired temp and I can make it directional. Tip: use the purpose built wire lamp not a regular heat lamp dome. The heater states using the wrong fixture will cause it to burn out.

Also Serpantmerchant says he uses hot glue to secure his probe. I would test it though.