It is a bit -you should have given them the Chelsea chop around the end of May-if you do it now you are unlikely too see flowers this year
I think the way it works is that if you do this while the days are getting longer you will get flowers that season, but if you do it after the longest day you won't.
I might be wrong, but I'm sure I heard it somewhere, but it might be a myth.
Anyway, I agree with Geoff, I wouldn't do it now.
Christopher2, that's interesting - when would you expect them to flower if you cut them back now?