im a strong believer in putting new tyres on rear and moving the rears to the front. That way your tyres are getting a good rotation and maximum life out of them. Whereas if you keep the same set on the rears for the life, then at one point you will have to buy 4 tyres at the same time, whereas rotation you just need to buy 2....cost effective.
even fitters for a certain bus company i work for, do this, move the rears to the front etc to get maximum life out of them and save money. Oh they use michellin tyres....and its michellin that send a "tyre specialist" to check the tyres and rotate as necessary.
EDIT: i was also informed that the law is 2mm (out ring on 20p coin) minimum for tread, however having 3-4mm is just the same as 2mm...especially in the wet!