Difference between determining non elective at end of plan year or per pay period

If a company may need to make a non elective to pass non discrimination testing at the end of the year does it matter in the plan docs if the non elective contributions are determined at the end of plan year or each pay period? I suppose per pay period would give them the option to do it earlier then end of plan year but wanted to make sure there are not other drawbacks to doing per pay period instead of end of plan year? 

Well, first thing that comes to mind is are there ANY allocation requirements? (hours, last day, etc.) If contributions re made prior to satisfying those allocation requirements, could be a mess. We recommend that if they want to "pre-fund" - they put it into a segregated employer account, then after end of year, when testing can be determined, and they have cooked their books to determine how much they can/want to contribute, move the money then.

It's also just a lot more confusing - more opportunities for error either with internal or external payroll, particularly if there is any excluded compensation category, etc.

I've seen a couple of plans that utilized per payroll, with NO allocation requirements and a design-based safe harbor flat percentage that worked, and I've seen a couple that were disasters. I certainly would not recommend it.

End of the year is best, as advised by both of the gentlemen above.  No reason that make sense to contribute a Non-Elective on a payroll basis.

Agreed....always better to use the exact W-2.

The pre-approved plan document we use has language in the basic plan document that a QNEC to pass nondiscrimination testing, whether or not a discretionary QNEC was elected in the adoption agreement, can be made. Generally, you wouldn't apply any accrual requirements (LDR, 1000 hrs) to a QNEC for the sole purpose of passing ADP/ACP. 

I have several clients that make employer nonelective contributions each payroll period.  The contributions are for a fixed percentage (not discretionary) and there are no allocation conditions once an employee is eligible for the contributions.  It is no more error prone than allocating matching contributions on a per payroll period basis.