If you're already up to practice exams, I'm assuming you've finished the course. If so, I don't see much point going to tuition at this point in time. I attend chemistry tuition and I find that it is somewhat useful, as it exposes you to the concepts and topics much much earlier compared to school. However, sometimes we would often only be slightly ahead of school (I have no idea whether this is because the teachers teach really fast or what) but now that we've gone over the course in tuition, we are pretty much at the stage you're at - practice exams.
So in conclusion, I reckon if you're doing well for your practice exams then keep going along with them, just make sure you mark all the questions you got wrong when correcting it and go through them again later on so you understand where you went wrong... I guess when you get your exam results back is when you can decide whether to take on tuition or not.