How often are you going to sleep and how often do you expect it to actually change?
Remember that the CONFIG words are in flash memory. If you're changing those bits often enough you're going to wear them out.
Datasheet claims 10000 cycles typical, if you're writing once per hour you're hitting 10000 writes after 14 months in operation. If you're writing once per second then, well then it's 3 hours....

/Henrik.