It is completely up to you if you want to stop supplementing before taking a test. If you are supplementing, by taking a test you will establish if you need to up or reduce your dosage but if you decide to stop supplementation before testing, you will get a greater idea of your baseline levels from diet alone.
If you do wish to find your baseline dietary level, we recommend a washout period of 2 weeks (Iron and B12) and 4-6 weeks (Vitamin D).
Always consult your GP or healthcare professional before making a change to your supplementation plan.