I think more important than having a good hand is what kind of odds
are you getting to make a call. When there are multiple callers then your calling range becomes very wide in the blinds
What else should i be looking for ?
Well things like what kind of player has opened and what is his position, what are effective stacks, your hand - based on this you should be looking for spots to play aggressive if you can like three betting, trapping or simply folding.
At what stakes does blind defending becomes important ?
every stake i guess. Unless you are in a tournament and you do not have enough stack so you cannot afford to defend most times.
how badly would it affect your win rate ?
depends how well you play at these spots. If you just keep folding unless you have premiums then obviously it will dent more as you would keep bleeding every round.
well there are so many variable involved that a paragraph or a few points is not enough. So look for strategy articles here or online and i am sure you would find loads of em.