Unraid parity has no filesystem and isn't journaled in any way.
Exactly correct. But, as long as you cleanly shut down, and have no hardware failure that writes erroneous data, it's reasonable to trust that it is valid.
If all writes were complete and flushed out of RAM cache at the moment, absolutely nothing, parity would still be valid. If, however, there are uncommitted writes to any data drive, then the parity emulated failed drive will be corrupt at the address of those writes. Sometimes that address is in unused space so no effect, sometimes it's in the middle of a file, resulting in just one file being silently corrupted, sometimes it's in the filesystem resulting in an unmountable file system that may or may not be repairable.
Until a correcting check is run, Unraid has no way to know the parity disk is out of sync, so any errors will accumulate, but will not effect you until a data drive fails to read a sector and parity is used to recreate that sector.
The longer you wait to run a correcting check after an unclean shutdown, the greater your risk of corrupted data. However, if the cause of the crash is unknown, you need to figure out WHY, before you correct parity. No point in writing bad data if the crash was caused by a failing stick of RAM.
Unraid is not designed to handle unclean shutdowns gracefully, so if your power is prone to failing, you must use a UPS.