I've heard this - the bell is designed to hear lower frequency sounds - but in my experience if you're gunna hear it you're gunna hear it with either side and if you're not you're not gunna hear it period. Plus there's the whole forgetting to open the right side... find something that makes you comfortable and stick with it. If you do use the bell remember that you don't want to auscultate with the same kind of pressure to the skin as you do when using the diaphragm.
(Just hook them up to your machine and get on with life.)