Some designs will run fast or slow when the battery is nearly worn out, not that the clocks in many systems are that good at accurate timekeeping to begin with.
Most of your other CMOS settings can be fine-tuned after installing a new battery, even if you don't remember to note their settings first.
The main one to watch is your hard disk type number, otherwise you may not be able to reboot after fitting the new battery. Most new systems will auto-detect, however, so you may not even have to worry about that.
