battery cycles

Apple says new notebook batteries are designed to retain 80% of original capacity after 1000 cycles, and last up to 5 years. (2nd paragraph)

What bothers me is these sporadic reports of battery swelling after a couple of years. Granted, it doesn't seem widespread, but enough that their should be acknowledgement of cause and effect. Users report that Apple is saying that the customer should have replaced their battery, of their own volition before the incident. What is left out is a cogent explanation of just what a user is supposed to use as a clear indicator of when that change-out should be undertaken. I've not seen the number of cycles discussed in these reports, and whether the user exceeded the rating or not.

What if your battery is at 600 cycles and it is down to 78%? My warranty is coming up. Is this a valid reason for a swap. In context, I have a MacbookAir 5,2 and it is on its second battery. Had to have a better part of the components replaced due to a faulty heat sensor (wouldn't turn on the fan, cooked the components).