I'm 'better safe than sorry'. If I can't tell, if it is that questionable (e.g. slimy lunch meat). It goes in the trash. On the other hand if it is canned or vacuum sealed, I always take a 'chance' because there really is no 'chance' to be taken. Its good basically forever until or unless air gets in there somehow. It will just be a little soggy. I have some tuna packs from "best by 2011" that I plan to eat well into the future if something untoward happens. Yep, I'm a prepper... to the extent I realistically can be.
Oh and, sometimes I do find things have actually expired before their printed dates. So it is always a good idea to really check first. Spoiled milk has to be the worst to encounter if you didn't at least give it the good ole sniff-test. Blegh!