Well, honestly it depends on the situation. Example, in a critical care unit or a step-down unit, I would explain that I will be assessing heart sounds, and if possible it would be best to go all around the heart, showing them, using my own chest. I am not a large person, but I know exactly what you mean about large breats. If they are consenting, you may have to displace the breast tissue in the opposite direction from where you are trying to hear around the precordium--the hardest spot may well be the mitral value areas--unless they are really, um, serious victims of gravity and major sagging is an issue. In that case it may be an issue of auscultating radiation to the apex. But with big breasts I find the TV and MV listening regions are tricky with these women--or even large/breasty men. I have sought to elicit their help with displacing the breast tissue, if they seemed like they would cooperate, and it was not an issue for them.
In situations, such as community health and other such settings, I may not be able to do a fully evaluation of heart sounds, b/c the patient may be fully clothed, and no really openned to a detailed positioning and listening on my part. I sort of play it my ear, but I usually strive for 2nd and 1st ICSs, LSB, RSIC, RSB, Erb's pt., MV-->L 5th ICS medial to L MCL, and lower LSB. Needless to say, moving into pediatrics has, for the most part, made this endeavor a bit easier. Just try to explain what you will be doing to the patient and assess their reaction and go from there. In many units, assessing heart sounds fully is expected and must be documented, and a fair number of folks therein are either sedated, still anesthetized, or comatose. But for the one's that aren't you will begin to find ways to respectfully direct your client, elicit their support, and assess. Ausculating heart sounds takes time--for the inexperienced and the experienced, it's just that the experienced knows more what they are hearing and where, etc. But to do it right, it requires a quiet environment, cooperation and good technique, knowledge, attention to details in sound, practice, and evolving skill. A good, thick cardiology stethescope is helpful too, as well as keeping the tubing straight so you can get a straight pathway (unblocked) for the conduction of sound to your ears. So the more cooperative the patient, the more quiet and the more tolerant of the minutes involved in your careful assessment of the major areas, the better.
Heavy/large breasts, bulky clothing, and bras intervere with the ability to accurately appreciate heart sounds. People listen over clothes to both heart sounds and lung sounds, but it's tough to really get fined tuned with what you are ausculating over any kind of cloth or clothing. Sometimes the obvious course crackles/rales may be heard over clothes, but you really can't do the job right over clothing, and this is doubly true for auscultating heart sounds. I don't like to go through the motions, so it's a pain for some folks, but I will go under clothing in order to hear properly--unless they are not amenable to the exam.