In a single word “yes”.  If you want your clothes to have the perfect fit, you have to wear the correct foundation garments.  Formal gowns have to be properly supported, each with there own special requirements.  Your day-to-day wardrobe requires a properly fitting bra.  If you want to create a smooth silhouette you might need to consider seamless briefs, a body shaper, or a slip (yes, they are still available). 

When considering what to purchase, think about the look you are trying to achieve and your personal fitting issues – we all have them!  While your foundations do not need to be  uncomfortable, they do need to provide support.  You also want to buy the best quality possible so they will retain their shape and not stretch out the first time you wear them.  If you haven’t been properly measured for a bra by an experienced bra fitter, put this on the top of your “to do” list.