When a tek calls for distilled water does this meen its absolutely essential or can tap water be substituted?

When doing simple kitchen style extractions from plants I don't think DH2o is necassary.

If it were synthesis it may be different as impurities in the tap water would react with reagents, but if your tap water is fit to drink then its fit for a simple extraction

If you have distilled or the reverse-osmosis water dispensed from machines in the supermarket then you may as well use it. Using deionized water is likely overkill.

Tap water is probably fine, but my beef with it is that its composition can change with the seasons and I don't know if stuff like chloramine (a commonly used water supply disinfectant which doesn't dissipate like chlorine does) can react with the soup of organic molecules being extracted--tap water may affect repeatability, or in the worst case generate chlorinated compounds.

there's no evidence to suggest chlorinated tap water alters products from an extraction.

That's good to hear... thanks.