A tuner is a valuable tool before practicing. It's important to start with an instrument that is in tune. In class we use A=440 as our pitch standard, so make sure that your tuner is calibrated correctly. Sometimes a tuner is helpful during practice, to point out slight adjustments needed. However, our main tool for tuning while playing is our ears, and a big part of every orchestra course is calibrating your ears to listen with increasing levels of refinement.