I think it really depends on the time of year and the type of athlete you are. Early in the year when you're building mileage the intensity of your runs should usually be kept conversational unless you already feel good. Later on once you've adapted to the mileage and workout grind then it's fine to start cutting down the pace towards the end of runs.
If you're a mid-D guy I would probably always keep long run easy -- as Gags says, "communicate with the birds." A harder long run, even one where the pace theoretically isn't super hot (i.e. what a pace calculator says you should be doing) can be a difficult stimulus to recover from while doing 2 - 3 800-1500 workouts a week. Better to keep the legs fresh for those efforts than to fry them on Sunday. No need to enforce a jog, just make sure the entire run is a truly easy and unchallenging effort.
If you're more of a long-distance guy coming at it from the strength end then I would say it's alright and even important to emphasize pace on long run.