I know this was directed at @Bird, but I'll add my thoughts as well.
@Bill Presson is correct that money in an account due to an uncashed check is a plan asset. It doesn't matter whether it sits in the plan's account or if the financial institution holds it in a different account until it clears, it is a plan asset.
In situations where the check is issued in December and clears in January, or we know that $xx.xx of trailing dividends will hit in January, I have no problem making the accounting work so that liabilities cancel out assets, and we can avoid a new plan year just for the sake of a slow clearing payment or trailing dividends. When those events start taking longer and longer, going in to February, march, or beyond, it loses any nexus it had with the prior plan year.
I would not treat a check clearing in March as no plan assets on January 1.
I believe it was mentioned in another thread that "client does not want to file a Form 5500 for an additional year". I think we all know that what the client wants to do or does not want to do in regards to reporting and disclosure is irrelevant.