Hmmm - I do not have anything in 'writing' about how to handle this cause its never been an issue - with me it comes off the next cheque - if parents have given me a bunch of post dated cheques and do not bring me a replacement to fix it - I just tell them I will change the amount on the cheque I have and then I give them the credit and ask them to initial the changes at pick up so that their account is balanced.
However NO I would not allow a client to HORDE their credits/over payments until the end of care or something where they all come off in one huge chunk on me that would make it hard to budget for - it comes off the next payment either way either - correct cheque if you do not do post dates or adjusted cheque if you've given post dates.