Depends on what you expect the total number of write/erase cycles may be for a typical life of the product. The manufacturers usually guarantee a certain number of write/erase cycles for flash cells, and the chips will frequently exceed that limit, but by how much - who knows? For many microcontrollers, this number is around 100,000 cycles. I would say that if you think that you're going to write a value more than ~100,000 times over the expected life of the product, that you will use a whole buffer area like I described so you avoid wearing out that one cell/byte - and then no longer are able to store updated values in it. What wears the flash out is the erase cycle (turning back into 1s) all the "0" bits that end up getting programmed when you write new value. You can only change a "1" bit to a "0" bit with a write, you must erase the sector of flash to turn the "0" bits back int "1"s.
