tantrums is a bit strong a word really - more like paddy's. My DS has never really had tantrums - I've been lucky! Lately he's started to stamp his feet and moan loudly when I say its time to stop playing and go for a bath for example, or he can't play on the computer its wind down time/supper time etc. They don't last that long and I tend to just ignore them.
He's also started saying no when I ask him to do something - he's always been quite amenable. Although he will (usually) eventually do the task. Is he just trying to assert himself and its just a 6 year old phase?