In our guild I'd say running at least one +15 is mandatory per week for the chest since we raid mythic. I know I've gotten some pretty good upgrades from the chest. Anything beyond that is optional but I'll happily run more to help guildies out on their alts etc if I'm available to do so. More AP for me, more chance of a legendary, potentially more chance of loot etc. It doesn't take long either to do a +15.
So unless you are raiding in mythic it's not mandatory but still nice to do if you want a potential 930 item.