I'd suggest people check their values every other month or so to make sure their values aren't too high.
Especially those taking higher grade units, some pills are >5000iu.
I recently spoke to my GP about this and was told they don't usually bother testing people for Vitamin D levels in the UK because most of us would be deficient. She recommended supplementing (I believe 4000 IU) all Winter and suggested possibly continuing throughout the entire year.
Edit: am I wrong? Ingesting too much fat will be bad for one's health, ingesting too much carbohydrates, minerals, protein will be too. The same goes for any vitamins. There's no _efficient way to store anything_ if you ingest too much of it.