You are required to notify all IRA participants of their year end balance. You are required to notify all IRA participants with required minimum distributions in the upcoming year of the amount of their RMD and the date by which it must be withdrawn. Both can be accomplished by sending an early copy of Form 5498.
For IRA participants in distribution, required or otherwise, there is a requirement to periodically remind them of their choices on withholding and point out that prior decisions can be changed. It does not have a timing requirement per se; i.e. it does not have to be sent at year end. However, it can be sent along with the information mentioned above.