Hi H&O,
The Period and Year end functions is Adagio Time&Billing are 'soft' as compared to other applications. What this means is that only YTD statistics are cleared when running Year End, as opposed to any records being cleared from files such as WIP. All statistics except for Budgets are cleared by Year End (Budgets has it's own function).
So unless the YTD statistics are important for your reporting (Files, Staff, Source, Disbursements etc), then running Year End is not a required step. Regardless of whether Year End is run or not, your WIP file will remain unchanged.
There is some information regarding Year End Processing in the Help file, in case you need to know specifically what happens.