You need to keep on top of your accounts receivable to avoid loosing lots of money if the client walks away.
In my contract it states the terms and conditions of sale and what happens when the client falls behind on payments.
-not paid within 10 business days will result in halting work.
-fails to receive payment and intends to stop work it will first attempt to contact the Client by telephone and advise the Client of such intention.
-The Client will have one business day to cure any such payment problem prior to ceasing to perform Services.
(BTW: never had to enforce the above yet)
They have to pay other service providers on time (e.g. hosting, domain registration, general suppliers, etc) to keep their account in good standing and not loose the account. You should be no different.
As far as loosing the client goes, they obviously don't respect you (or any other people they are behind on) or they would have contacted you to explain why they have fallen behind. Avoiding the issue just makes for bad feelings on both sides.
Don't release anything to them until the account is current.