Even if you’re following a tried-and-true recipe, you should still taste your food as you cook it. A whole slew of other variables can arise during the cooking process, and you need to know what’s going on in that pan or pot. Your mouth is going to tell you what’s what. A metal spoon, however, is a potential hazard.
As an unapologetic craver of the glorious mineral that is sodium chloride, I firmly believe that anyone who's serious about cooking needs an easily-accessible salt cellar. No other ingredient changes the way you perceive flavours like salt does; while it's rarely the sole seasoning agent in a recipe, it's the most important to get right.
This is not because metal spoons can’t get the food to your mouth—they can—but they are far more likely to burn it than a spoon made of wood (the best spoon material). A wooden spoon does not retain heat in the same way, nor does it heat up as quickly as a metal one, making it a much safer choice.
“What about silicone and such?” you are undoubtedly asking. I have found that the slippery nature of silicone makes it bad at holding onto food, which is annoying when you are trying to get said food to your mouth. (But if your only option is plastic or metal, by all means use the plastic.)
Though tasting with a wooden spoon will greatly decrease the chances of you burning your mouth, it can’t eliminate the danger completely. Boiling liquid is boiling liquid no matter what you scoop it up with, so use common sense, and maybe wait a beat or two before pouring it on your tongue. Tongue burns are extremely inconvenient.