Rinse meat after marinating in yogurt

I can't see any reason for the marinating itself to make any difference. If it's safe to leave the (un-marinated) meat in the same conditions for the same length of time, then it's safe to marinate it for that long.

Five days in the refrigerator is definitely stretching it for chicken - usually no more than a few days is recommended, and that's assuming it was fresh when you bought it (not a day before the sell-by date). Even if it's safe, it's not going to be very good. I've accidentally left raw chicken in the refrigerator and forgotten about it before, and it started to smell "off" after about 4 days. I hate to say it, but I wouldn't use it at this point.

As for the steak, you're probably okay, because you only have surface bacteria, but I would cook it well and right away. Again, most cooks I know will recommend no more than a few days in the fridge.

P.S. I've heard people say that weak acids such as lime juice "preserve" the meat, but never from a reliable source; even if it worked, every marinade is different and it would be nearly impossible to predict the exact amount of time it preserves for. You have no idea how much bacteria existed at the outset, and even if the marinade somehow helped to inhibit growth of new bacteria, the "spoiled-ness" of raw meat doesn't come from the bacteria itself but from the toxins they leave behind. It's not a requirement for the bacteria to multiply in order for the food to spoil, if enough existed in the first place.

Unless somebody has it on good authority that marinating makes any significant difference, I refuse to put any stock in that bit of folk wisdom and recommend that others be equally skeptical.

There are several reasons why you should marinate before cooking:

• Many marinades contain raw ingredients that should be cooked along with the food being marinated, such as garlic or ginger. In some cases this may actually be a health hazard (raw garlic can harbor botulism), in other cases you'll simply end up with an undesirable pungent flavour.

• Many marinades also use some amount of acid, which helps to kick-start the process of breaking down the connective tissue and tenderizing the meat. This is an especially big deal for chicken breast meat and tougher cuts of beef or pork, because most cooking methods (except for slow-cooking) don't do much of this and tend to produce a tougher, chewier end result. Once the meat has already been cooked, using an acid-based marinade won't give you much more tenderization; you need the combination of acid and heat.

• Finally, many spices release most of their flavour when heated (cooked). Examples are cinnamon, saffron, star anise, and various types of hot pepper or chili powder. If you marinate after cooking, you won't get much flavour from these.

Of course if you are using a very simple marinade such as soy sauce and are marinating only for flavour purposes, then it might not matter much, but that's the exception rather than the rule. Marinating afterward can certainly improve the taste, but most of the time you'll end up with better results by marinating beforehand.