I am referring to the penalty when the total of withholding and estimated payments are not paid in by end of year (or filing date?)  I have looked at the form instructions and they seem a little vague in that they do not state that they must be paid by year end, filing time, or January 15...? (since this is the final quarterly estimated due date for the prior year...?) 

Is the 90% rule the one you are referring to for not enough paid in at filing time? If so that answers my question.  But even the instructions can be very confusing - bottom line for me is - should I tell my client to have at least 90% paid in by Dec 31, or when...

I appreciate all help and answers.