I call the clients with balances due explaining I need 8879s to efile or there will be penalties and interest, etc etc      Like @IRonMaN ,  I don't want to have to deal with IRS notices.  Clients conveniently forget they were the cause for the late filing when the notices start arriving.

I don't call clients with refunds.  When they call to say - "hey I haven't received my refund" - I can say "hey, you didn't return the 8879s"      They don't forget the next year.