We can't answer if YOU should take beta blockers, treatment is individual and must be done by your cardiologist. That said, beta blockers are often used to prevent arrhythmias in the setting of LQTS.
There are a lot of drugs to avoid in this setting. You should trust your doctor, and ask your doctor before taking any non prescription drugs or other supplements (including herbs, etc). Also, be sure to inform all your doctors that you suffer from this condition, especially before taking antibiotics, allergy drugs or anti anxiety drugs, which often affect the QT interval.
Also remember to seek immediate medical assistance if you feel severe palpitations, fainting, etc, and see your doctor if you experience an increase in PVCs. As you probably are aware of, LQTS can predispose for a serious arrhythmia.
You should talk to your physician before taking aspirin if you have LQTS. Antihypertensive drugs are on the long cautionary list.