I often go but find it depends on the fitter themselves. Most seem to get my size well out and end up going through the rigmarole of faffing around in sizes until I'm proved about right.
M&S sometimes do an offer if you go for a fitting though you often have to book in and come back later, depending on the store I guess but I go to a busy one.
I found the Debenhams fitter I saw last time was very good. Bravissimo staff are usually always great though, again, you often have to book in for a fitting sessions.
I was taught the tipping into the cup thing some years back by a very good fitter, at a G cup though I have to watch I don't topple over ;)