It is far more important to look at the patient than to look at the ECG.
You can have patients who have fairly normal looking ECG tracings, but are actually needing immediate intervention. On the contrary you can have patients whose ECG's look erratic and worrying, but they are fine in person. This could be due to e.g. a lead falling off of the chest or the patient coughing or sneezing while the tracing was being conducted.
In order to read the ECG correctly the speed of the tracing must be set to 25mm/s and the gain set to 10mm/mv.