Is it better to use combat scanner probes in the initial stage of signature detection and then swap over to core probes? Or is the base maximum deviation for combat scanner probes too great to allow for a fast lock on?
Not Telling!
It may be worth firing off a combat probe if the system is bigger than 32 AU. (Technically, if the system is bigger than 28, since signatures will be within 4 AU of the outer bound of a planet orbit) Otherwise it's a waste of time since a Core probe will scan with less deviation and better strength. But if you do, you'll still almost certainly need to switch to Cores at some point to lock down a signature, which is recalling and reloading the probe launcher... kind of a waste of time.
Or, if you're looking for people or Anomolies. Then a combat scanner makes sense..

All answers were very useful, so I up voted you all. However I feel that Selari, and in my recent experience, is the most accurate in answering the question.