Not at that level. 490 defense is one of the last stats you'll want. At least it was for me.
Pre-70 = Stam > +spell damage > defense > STR/INT (toss up)
I had near 10k un-buffed HPs at level 65-66 & a decent amount of +spell damage. Hardly no +defense & i didn't even look at my avoidance. Some runs were tough, but I never had a problem with dungeons. Actually, with Ardent Defender & all my HPs it was probably easier on the healers (which were NEVER healing spec -.-).
You can get very good gear though questing for tanking pre-70 stuff. Every now & then maybe pick up something from the AH if it doesn't look like it'll be replaced anytime soon.
Shield of the Wayward Footman comes to mind. But watch yourself here, it can be tough - as in it's rare to see good tank gear up at the AH around that level, AND silly - as in you'll find something, drop 20g, then replace it 3 hours later.
As you level keep an eye out for good quest items. Most will be obtainable once you hit 69-70, but few are earlier.
Breastplate of the Warbringer is ideal for young endgame tanks. When you get to Netherstorm & Shadowmoon Vally you'll start to see a lot more +defense/+avoidance quested items, and then some nice drops that accomplish the same. THIS is when you should worry about 490d/102.9a.
Jumping into Heroics once you hit 70 isn't the brightest idea, unless you have a guild that can farm you your gear in them. If that's the case your better off healing until you can hold your own.
Good Luck!
Kz