The only problem with waiting till you see fleas is that for every flea you see,there are lots more eggs. Mine get Frontlined about every 6 weeks at present,as they dont leave the bedroom much less the house (I frontline with fostering in the house), but I do only worm them 6 monthly now, due to not going out - the closest Molly gets to hunting is chattering at birds through the window!! The main reason for the change in recommendation is for people who have children, as obviously humans can get worms from pets, and they are more likely to be at risk