Should I look at nominal or max battery voltage for choosing resetable fuse?
Maximum since that is the voltage the fuse might have to interrupt. Voltage ratings don't play any role while the fuse is conducting. In your case though, you probably won't notice a difference in practice due to tolerances and battery voltage dropping as it is discharged. It's not exactly difficult to interrupt 16V or even 24V. Now, 100V or 200V or 1000V...that's different.