I don't remember rats being unsafe to eat KC. I've eaten rats of every variety without poison resistance and have never suffered ill effects. Wererats are an exception though, they'll convey rat lycanthropy (a not cool ailment to have) and, if you're a human, counts as cannibalism, which'll net you the aggravate monster intrinsic (the opposite of stealth, basically) unless you're a caveman.
EDIT: I just looked it up, and every type of rat except Rabid rats are safe. Rabid rats are poisonous, don't convey poison resistance, and shouldn't be eaten.
But yeah, don't eat any corpse after it's about 50 turns old. 50 turns is a very short period of time, and food poisoning is fatal. Food poisoning is not the same as regular poison, any old corpse can give you food poisoning. If you DO contract food poisoning somehow, pray immediately, and your god will help you if your prayer timeout is zero. The exception to this rule are lichens, which never rot or cause food poisoning. They're an inefficient alternative to rations or lembas wafers though.
Never carry around corpses either, most are very heavy and will fill up your weight allowance very quickly.
Ontop of what KC said, you can avoid hunger by wearing a ring of slow digestion, a fairly rare ring that greatly slows the rate at which you consume nutrition.
Also, there are a number of "poison" creatures which'll poison you and possibly lower your total strength or kill you outright. Killer bees, homunculi, giant cave spiders (normal spiders are alright), scorpions, soldier ants, giant beetles, and pretty much any non-garter snake fall into this category, and shouldn't be eaten unless you have poison resistance.
Congratulations Quincunx, you've done alot better than me. Just curious, what'd you use your wishes on? and what did you genocide?