Well, you want to be flexible and accommodating, but you want to cover yourself from scope creep and you don't want to set the precedent that they can change anything whenever they want. I would probably do small changes for free, but call them out as exceptions, like "Sure, I think we can slip this change in under the radar, without affecting the project timeline!"

tptacek posted a great line the other day which I stole for my personal use: "I am happy to do whatever you'd like and make whatever changes you'd like but I need to remind you that we have a fixed schedule for this project". This line concerns scheduling, but I think it could easily be adapted to deal with scope and changes.