Me I always switch em on when cleared for takeoff and also when entering the zone. they go off when I've left the runway after landing or when I've left the zone outbound...if I have to cross an active as part of the taxi then I turn everything on while crossing. For those interested here's what IFALPA has to say on the subject :
http://www.ifalpa.org/downloads/Leve...ion%20risk.pdf