I speak up as soon as they take me to their chair. No point in being disappointed later on. If I don't want to be trimmed, I speak up before they even reach the scissors. Or better even, when they ask, when was my last one. I'll say, it was on X date and I will not be getting one this time. You have to speak up from the beginning. Sometimes they can be put off because they are used to running the show, but they get used to it. I don't know why so many people don't speak up. It is my hair and I am its only advocate at the salon.
I remember when I was seeing a stylist in Philly for the first time. He asked me what kind of relaxer I was using and I said regular Affirm--the kind you don't mix up. He tried to act like he didn't know what I meant. I said I use lye. Then he said, they all have lye. I said, that may be true, but I use the one that you don't mix and I don't feel comfortable changing. Luckily, there was a tub of it sitting in the distance and I pointed to it for emphasis.
After I was relaxed (with lye) he proceeded to blowdry me and I expressed to him at that time that I wanted very little hair trimmed off. I asked him to "be nice." He styled my hair wonderfully and was never scissor-happy. I stuck with him until I moved and he never let me down, because I ALWAYS told him what I thought or wanted from DAY 1. I am like this with all stylists.
Better even, if they start to see you as a "picky" client, because then the roles get reversed and they are "scared" to do anything to you without your permission.
I've seen stylists react and even comment under their breath when these types of clients come in the door, but they always do what they say. I'd rather be seen as a pain, so long as they do what I say, rather than staying quiet and leaving jacked up!
Just remember that you are your hair's only advocate. Establish your stance early on and you can have a great relationship with your stylist. I can safely say that I have not ever had a communication problem with a stylist because I speak up and tip well when they listen.
And I don't mind walking out with a wet head if I sense that I can't trust their judgement...