Personally, I go by what works for me. I generally follow the mrmalty calculator, except sometimes I pitch an activator pack to a ~1.050 beer and it turns out just fine. I don't make a starter for anything less than 1.040, and that turns out fine too. For many styles (belgian and hefeweizen), I make a smaller starter than recommended, as I want to stress the yeast more to get more of the yeast derived flavors.
I also totally "overpitch" by pitching to yeast cakes for larger beers >1.070, or any lager over 1.055.
When all is said and done, I think you'll find that yeast tend to be very forgiving, and they've always made beer for me, sometimes better than others, and I learn from my mistakes. Do what you're comfortable with, and what works for you