Doesn't that require the clients letting you know when IRS will receive their payment?  If they couldn't get it to IRS in time when required, why trust them to come up with a date now.  Also, if "the check is in the mail," how long will it take IRS to receive it and cash it?

But yes, sometimes I do that, and include it on the return.  But the software doesn't do the computation for the state, so I either have to eyeball that or tell the client to expect a notice. 

Still drives me crazy to see other preparers including ES penalty with the return, when less than $100.  Saw one the other day where it was $22. I think they charged $25 for the 2210.