Guitar tuners are typically designed for tuning an instrument with 12-tone temperament and would not work with your voice. This means that each pitch is at the same distance from the other, and this might not help your intonation and lead to unpleasant tone colour. Using a guitar tuner for singing can also affect your ear practicing and flatten your voice further. But if you’re just using it for practicing and checking your pitch, that should be fine. You can check if your pitch matches the one you’re set to achieve with the help of a tuner.