Yes, you should definitely use both, I tend to be lazy sometimes and use the open string whenever I can. But there is an advantage to playing the open string as often the playing of the open string sets of sympathetic ringing of other strings (which is common in folk fiddle playing and not in classical) Its interesting that your teacher has a cape breton background as its a very common ornament in cb style to play both the 4th finger and open string at the same time. Also you need your 4th finger on the e string (for the b). I cant stress enough how important it is to keep your hand straight (rather than flat like it's supporting the fiddle- your hand should not be used to support the violin you should be able to do that with your chin and shoulder a good shoulder rest (eg Kuhn) will really help. But do whatever you want of course. Have fun and Keep fiddling. Petr.