With an object it is always necessary. But if there's no object, it's not used. Like Tittar du eller inte? 'Are you looking or not?'
på in this case is a preposition so it's not stressed. To make things more complicated, there is also a particle verb tittar på where på is always stressed. This verb has a slightly different meaning. It cannot take an object, but it always has the particle. So with this verb, it would be Tittar du på eller inte? 'Are you watching or not?' – the difference in meaning is very small in this case.