Thanks for your replies.
The legal requirement ICAO and JAR (and I believe FAR 91 as well) is to have nav/psn lights from SS to SR. In fact MELs usually permit dispatch with them inop in day time.
On my previous airline we were advised to turn them off in daylight/VMC on the 757 (I think they run on expensive inverters). On other types (Douglas) you keep them on all the time and the strobes start flashing when you rotate. On Buses they operate in conjunction with the logo lights.
Many German airlines (LH for one) keep them off during the day, and so do many British aircraft.
Our current SOPs just say "as req'd", so I'd like to know about your company policy.