Should I evolve this Dratini or hold off?

Depends on your priority -- to get that perfect Dragonite or to just get a Dragonite while spending minimal resources (candy and stardust). IV is only important if you want to hold top spot in the gym. Otherwise any IV of Dragonite is worth evolving.

Evolve, that's a solid IV! Unless you live near a dratini nest or happen to get 10K eggs and hatch dratinis frequently, go for it now! That's plenty of candy to level it up to be a solid monster. Good luck on the moveset!

Evolve it.

What difference is 6 extra defence going to do for a Dragonite that already has really good stats. Damage is rounded so it's unlikely to make a difference to how much HP you lose from every attack. Once in a blue moon it'll reduce the damage you take from a quick move by 1, which does add up, but potions are plentiful and gyms are easy to take down even with less-than-optimal pokemon.

If it gets DB/DC or DB/DP I would personally power it up, because personally, I want a pokemon for taking down gyms faster with. I don't hold gyms, I just dump a Pidgey or whatever, collect, and transfer when it faints, so I don't have much use for a defending Pokemon.

The way I see it, gym placement doesn't really matter when gyms are too easy to take down. It would matter if gyms were only losing some of their prestige but not all of it, so that the lower pokemon are constantly being kicked out and others are adding in theirs when someone trains up the gym again, but that never happens in my experience.