Additionally, using a magnet holder can magnetise your tweezers. My go-to tweezers are a fine Triumph watchmaker's tweezers made of carbon steel, and as it is, they will magnetise slightly over time. This makes it very frustrating to handle small parts made of iron/steel, for example, a carb linkage made of .009" guitar string, or a plastic door handle with guitar string pin, etc. I find I have to de-mag my steel tweezers about once a year, or so. I would expect a magnetic holder to aggravate this problem. Not a problem, of course, if only handling plastic, brass, and aluminum.