I'm sure they all follow suit.
I change my supplier every 12-15 months (using cashback deals too) but unless you take a fixed rate deal (which can often start out more expensive) you can find that the price of your cheap deal goes up within a few months.
The price comparison process would be much easier if companies all had to set their prices at the same time- maybe at 6 monthly or 12 monthly intervals. I think Ed Milliband's proposal for fixing prices will backfire in some respects as I think energy suppliers will try to get their increases in early, but I think he has made people realise that much tighter regulation is needed and the market isn't anywhere near competitive enough.