This is not true, I have been using socks now for a good few months and every time I do my testing results are bang on and a added bonus my tank is very clear.
Like any other piece of gunk absorbing equipment in your sump, if you don't change/clean it regularly it will cause problems. I use them and have never had any ammonia or nitrates in my tank but I also change them every 4-7 days.
yep I use em and change it out about once a week or every 3-4 days. I have 2 that when I take old one out I just throw it in the washing machine and run it with out soap and comes out clean and bright white again.