Totally depends on whether there are any special events, shows, concerts etc on the day you are going. For instance, we are eating twice at Epcot during Flower Power concerts. I therefore have dinner 1 night at 4:45 pm so we can make the late show and not until after 7:30 the other night so we can catch the early sets.
I have never tried to make an ADR to watch Illuminations; eating that late just does not interest me. Epcot is so big that I don't feel compelled to try to stake out a viewing place hours ahead of time, though, so I wouldn't be eating at 5 pm just so as to go stand around waiting for Illuminations.
Bottom line is that unless there is some specific show you want to see, schedule dinner for whatever YOUR normal hungry time is. Yes, 6 pm is prime time for everybody so ADRs for that time slot do fill up first, of course.