As others have said, the wagon wheel drill is a very good friend for teaching a dog to move with you and to get an initial line. I also agree with Wayne, the looking around is probably some form of bugging (avoidance) that could come from a number of factors, such as the dog is unsure of what to do or has had a previous bad experience.
To answer your original question on when to cue, I believe that the actual word you use is the least important cue you use on a blind or mark. I know Rex Carr very much believed in few words being spoken. The pro I train with has taught me a lot about differentiating between marks and blinds by voice inflection and line movements. As for voice inflection, he taught me that you give a dog permission to get a mark and you demand they get a blind. Also on the line, you cue a mark by not doing a lot of moving once the birds go down or on the return for multiple retrieves (you get the "first look") as Pat Burns describes. On blinds, especially mixed in with marks, you generally re-heel the dog as a cue. If a two-sided dog, sit them, then heel to the other side and line on the blind. If a one-sided dog, heel back a step and then forward and then line on the blind. These, in my opinion, are actions that a far more important than the dog understanding the word "dead" or "mark". That said, I do use the words after the dog sits, but only so I can use voice inflection on the words as a cue.
One more illustration on how this works on a different manner. Think of being on a honor. You may use the word "no" or "no bird" or "leave it" or something like that. But I bet the word is said forcefully or sternly and you probably (or should) use a different stance than when you are running the dog. I turn 90 degrees to the dog and look down at her eyes. Do you think the dog is cueing off of the word you said or the vocal inflection of the word and your different stance?
Hope this long-winded explanation helps you a bit....good training