Being wooden, guitars have a bit of ‘give’ to their structure, and as you tune up and vary the tension on one string, you are at the same time changing the tension on the other strings, changing their tuning. It is therefore a good idea to make several passes through the six strings, tuning each just approximately on the first couple of passes, and then becoming more detailed and exact on later passes. This applies whether you are using an electronic guitar tuner or not.
The Beat-Play Experiment 