Cue points are no more or less than reminders in a track for specific spots/events that YOU think could be useful to YOU. As such it depends highly on the kind of DJ you are, the kind of music you play and the kind of mixes you want to make.
You can set at the beginning of breaks, where the vocals start, at the end of a lengthy and boring intro or even the introduction of a nice instrument bit that you want to use somewhere.
There really isn’t a guideline to placing your cue points imho, because it’s so highly personal.
Personally I know most tracks I play regularly to such an extent that I really know what is where in the track. I usually just needle drop to a point I need and cue up. So I usually only set cue points on new tracks. When I listen to them after adding them to my DJ software (max. 5 -10 new tracks a month in my case), I decide what points are interesting to me and drop a cue point.
One thing I sometimes do is drop a cue point at the END of a section where I need something else to happen, like ending a mix or starting a mix. This way the cue point just acts as a visual reminder in my waveform that something is coming up.
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