There's a point where it has to go both ways, I suppose. Even if I use all my defensive cool downs, and Abyssal Drain, if my healer can't back me up when I need it, none of my cool downs are going to matter, which is where the trust in your healer being able to do their job comes into play.
I can trust in my defensive abilities all I want, but that will only get me so far. I've had healers in ilvl 200 gear do better than those in shire gear. Because they took the time to learn how to adjust, which means I can trust them to do their job, so I can do mine.
I just go off that I very very rarely die in dungeons, and when I do it's almost always been my fault. And I run enough dungeons that to believe I just get lucky and never get bad healers would be outrageous statistically. Maybe it varies by data center but walking with confidence that between me and the healer whoever they are we'll live has always served me well.
Unless the dps take a year to kill everything... after a point it's on them not the tank or the healer.