For the last couple of years, I've been working with board ops that don't have a lot of experience (acting students, interns, etc.). So, I've been calling warnings but not standbys. About 30 seconds before a cue sequence, I warn everything that happens in that sequence (I've been defining a sequence as a group of cues that happen within 30 seconds of each other - I haven't been calling very difficult shows).
I like it because it gets the operator's attention on the show instead of on their book or script or text-messaging or DVD or whatever, and it lets them know how far they have to go before they go back to their pastime. I do try to warn in clumps as opposed to chron sequence (I know some SMs don't like this), for example:
Warn SQs 160 thru 225
Warn LX 26 thru 32
Warn Trap Down
Warn Rain on & off
Warn Spot 1 on E at rain grate
If the Spot in the above example has to wait a minute before their cue, I may take a second to say "Standby Spot" immediately before the cue but otherwise, the ops just hear this gruop of warnings and then their cues. If the cues come faster than I can speak, I'll omit the cue number from the call but check the cue numbers at the end of the sequence:
Sound Cue 160 and Elex 26... Go.
Sound Cue 180 Elex 27 and Trap Down... Go.
Rain... Go.
Standby Spot.
Spot... Go.
Elex 30... Go.
Sound Cue 200... Go. Sound... Go. Sound... Go. Lights and Sound... Go.
Sound, that should have been Cue 225 and Elex, we should be in Cue 32.
(So far, this is my favorite cue sequence of all time, if you include the Wall Falling cue that happened a minute before the warnings)