The best thing you can do is discuss the procedure with scripty and sound before the start of the day. Sometimes the sound guy is doing "single-system", which means he/she is monitoring the sound, but sending it to camera. In that case, the camera see's and hears the slate clapping. Wether you call out the scene/take is up to you and scripty, but it's redundant. Also note that "single system" can also be the sound guy/gal recording to a Sound devices (or Dat...etc) and not sending a signal to camera, in which case they might need you to call out the scene/take #'s for slating reasons in post. Sometimes they have a mic attached to their recorder and they call out the slate info, so you just have to call out "marker"
Make sure you hold the slate completely still when you clap it. Hold for half a second and then get the hell out of there!