You should never have significantly more tread on your front tyres, Toureman's advice is spot on. I've juggled around with tyres in the past, in the interest of evening up tyre wear and, with one particular vehicle, ended up with snap-oversteer in a predominantly understeering car the first time it rained. This extreme can also manifest itself by using different make/type of tyre front and rear, so again, make sure you have the make/type with the best wet grip on the rear.