They always go on about stuff like this in diet magazines and stuff. Chewing thoroughly, stop eating before you're full, drink water, don't drink water and a million other tips to stop you putting weight on. It's all a load of crap, eat less, excercise more or you'll get a fat arse, them's the rules.
And seriously, have you tried to chew most stuff 20 times? You'd be dribbling trying to keep it in your mouth and not swallow by that time.