When making batter/dough, why is it really bad to clean your spoon by hitting it on the edge of the bowl a few times?
Oh, those cooking myths! 

Whenever you think you have heard them all, there's a new one. In a yeasted dough, the yeast is perfectly fine with being tossed, beaten and generally mangled. The little yeast cells couldn't care less about what you do in the initial stage of mixing and kneading. (That's obviously different when you consider the dough after the bulk raise: You want to keep the existing bubbles, sometimes more - think baguette or ciabatta - sometimes less, when you punch out the larger ones for a smoother texture, but I digress.)

What is an issue, is the kind of air that didn't come from yeast digestion and is safely entrapped in a gluten network, but the air that was beaten into the batter mechanically. In the oliebollen recipe I have, you make a fairly liquid yeasted batter and finally add beaten egg whites. Now, thats a material that doesn't take well to being jostled around. Just consider how carefully the egg whites are usually incorporated: There's even a cooking term for the method, you fold them in, instead of stirring like crazy.

Many bakers will handle a batter with incorporated egg whites very carefully, because they don't want to burst those precious bubbles1. But unless I am working with something super delicate, a gentle tap of the filled pan can help too-large bubbles rise to the top (and subsequently be pierced) instead of creating unsightly cavities or worse, darkened patches on the surface.

The kitchen rule of "don't bang the spoon on the bowl" may well originate from there - but then the rule applies to all kinds of batter with stiff egg whites. You can be on the very safe side and desist, but I am quite sure that it won't really matter if you tap gently. On the other hand, wiping the spoon with your finger or a spatula means you get all batter in your bowl, and not in the sink, which the frugal me appreciates.

Not tapping the spoon will also prevent chipping the mixing bowl, which in the past was often made of clay or ceramics, and avoid splattering.



1 That's something that fires back in macaron making, where you have to purposefully destroy some of the bubbles...