I can't honestly say that I noticed a difference, but I went from using a whole tablet per 5 gallons to a half. The reason I did this was from previous discussions about the possible effect on beer pH.Or just toss the whole thing in and don't worry about it.
From my experience with dark beers, I much prefer one that has been cleared/fined. The muddy look is not very appealing to me in a Porter or Stout. I think I mentioned it previously, but I still use whirlfloc in hazy beers. I prefer hazy over murky...So this begs the question, particularly for porters and other hard-to-see-through beers (and ignoring purposely cloudy beers like hefeweitzens): Does hazy beer taste different?
I use a half tablet in every 5 gal brew, with the exception of my Hefeweitzen.