I agree with this, and someone has already pointed to the Paul Graham essay "Do things that don't scale". I will say, you need to build to accommodate some growth, but in my head at the startups I've been at, I always figure we should build for 10 times current traffic. Room to grow. There were times we never got to 10 times current traffic. (and died). And a time or two we did, then it was time to re-architect for 10 times the new current load. Lather, rinse, repeat.