You can only get special resources if theyre within your borders, you have the appropriate terrain improvement (mine/pasture/winery/etc) and if you have a road to them. Correct? Given this, how far will you go to secure a new special resource? Build a city on a worthless island just so your culture gets there? Build a city on tundra by the ice cap? etc.

I think it really depends on how much you need that special resource. If it's the only iron you can get to I'd say that's pretty important. If it's a lux like furs, I probably wouldn't bother since there are other ways to increase happiness. A small city on bad terrain might be a drain on your economy, but doing without something like iron is going to be even more detrimental.

Iron/Coal/Oil and to some extent horses are all important. If you don't have iron by the time Barbarians do, kiss your sorry butt goodbye.