what happens if you take a new customer on a few months before june ?? ... guess you forewarn em ?
nice idea btw ... giving some thought 
If I take a customer on any time between January and June, I put the price up the following year.
I've had a quite a few potential customers who I've given a quote to decide against having their windows cleaned when I've said about the yearly price increase but as I'm not short of work, it doesn't really matter.
I've also had a few customers at the lower end of the price range change from 4 weekly to 8 weekly after a few years.
By and large though, it works really well.