i have a de who is simular built to another de..the difference in builds is his guy has 37 conf. and mine has 19 conf. he had 25 sacks and i had 7 against the same skill players and teams. so i believe it's actually very important!
I know that you have to at least have some. 1 missed tackle, 1 pancake and your dude will be toast. Confidence is more important that you think because it's more in the background. It's VERY important if you are lined up against higher level opponents.
If you're a speed/agility DE with low strength it's important, since you will get pancaked a lot and spend the rest of the game sullen. The other option is soft capping strength.
I agree with DiMo completely, but think that skel might be right that 30 is enough for a 20-35 lvl player.