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I've heard different things about it. And can't realy make up on my own what might be the best thing to do.
The question is simple... Should I coock the wood before placing it in my tank, or better not?

Depends on what wood you have.

If you have a light wood that won't sink, then boiling it will help waterlog it so that it sinks. If you got the wood from outdoors, then boiling will help sterilize it. If you want all of the tannins out of the wood quickly, boil it. If you have a denser wood and don't care about removing all of the tannins or sterilizing it, just soak it in a bucket for a few days and change the water until the tannins are removed to your satisfaction.

Yes, that's always the best thing to do. It helps to kill all things you don't want in your tank. Moreover, if you have roots (kienhout in Dutch) they will sink much faster when boiled. If your piece is too large to be put in a pan, you may put it in the bath tub with water as hot as possible. If you don't have a bath, put it in the shower.