i have a 3 year old. i always check mine (and have traveled with it many times). the airlines used to give me a large plastic bag to wrap it in for free. a few months ago continental charged me $5 for it. this past trip (we flew to italy about 4 weeks ago and returned yesterday) they didnt have the plastic and it got checked "naked" and came thru just fine on the way there. the return trip they lost the carseat. without the bag, it went thru "oversized" check-in and didnt get on the plane. if i had actually purchased the britax luggage bag made special for the carseat instead of cheaping out and expecting to get the free plastic, it wouldve made it on the plane with regular luggage. its not too big to be checked with the rest of the suitcases.
i dont know what the CARES are - will have to look into that. my son just sits next to me - i put him by window, never on aisle since i fear something from overhead will fall out on him - and i use lapbelt.
long story short - i recommend purchasing the manufacturers luggage bag for carseat. i am going to buy it tonight for my next trip!
good luck!