I have an opposite view to bud540, because I just don't like inhaling toxic fumes: the effects can stay with you for a long time (like forever, for instance). Fumes are sometimes difficult to avoid, but this situation isn't one of those cases. I have always cleaned valves by scraping them with a knife blade. If you keep the blade at an acute angle to the stem, and always move it parallel to the valve stem (i.e. don't let the blade edge run across the valve) you will not do any harm to the valve (but watch out for harm to your fingers). Some people use rotary wire brushes for the job, but I've found difficulty in penetrating really hard carbon deposits that way - the knife blade gets under them or just crushes and shatters them.
I use much the same method for cleaning cylinder heads and piston tops, but with even more care, since aluminium is easily scarred by a knife blade. (I actually use a scraper, made from an old file.) Nearly all the motor mechanics I've known have used rotary wire brushes for these jobs, though it is necessary to be gentle: a wire brush that runs across the sealing surface of an aluminium cylinder head would cause a real mess.
