You are right that the expiration date is a guideline, with that I do follow the expiration date very closely on any dairy item package. I am very sensitive to food so even if it is good but getting close the expiration date I can taste a difference and toss it.
With food that has been sitting out (that needs to be kept cold/warm) it should not sit out for more then two hours. Around two hours it can start to have bad bacteria grow on it. To me chicken is a touchy situation, I have gotten sick from bad chicken because it sat raw at room temp for too long even after cooking it I get sick. Now I have seen/heard that you can eat raw chicken if your stomach is adjusted to it, same with eggs, but most are very sensitive to it.
Leftovers should be there no longer then a week. In our house they get eaten for lunches with in a day or two so leftovers do not last long. Anything that I have room to freeze, I freeze, and it will keep longer.
We were throwing away food more then I wanted so what I did was buy less and go to the store more. Freeze all meat or anything I could because then it last about 6 months (I take out tomorrows meal out the day before and stick in fridge) so I can buy 2-4 weeks worth at a time. Canned food last longer, but some canned items to use in the month. Anything fresh (fridge, fruit, veggies, bread and so on) I get every 6-7 days so little to no waste.