In great celebration of me being a "Senior Member", I can help LOL. I learned to try to keep the vocals sitting as close to the center as possible, if not all the way in the center. Your main vocal should always be centered. When doing harmony, you can slightly pan the harmony vocals depending on how many vocal parts you are harmonizing. For example, if you have a main vocal and two near identical adlibs, you can pan one set of adlibs about 50% left and another 50% right to get sort of a fat stereo effect. Always try to picture your vocals on stage at a Mariah Carey show with no microphones. Mariah is always the loudest and the most up front. The backround vocalist are usually kind of stored away in the backround, but still heard in stereo. Keep this in mind, because harmonizing vocals is similar to being at an accapella show.
You talked about talking into the microphone and your vocalist getting a bassy response. This is due to something we call "proximity effect". Most Shure mics, especially their dynamic mics have a high proximity in my opinion, compared to other mics I used. You have one of their better condensers so its a little better. This can be good sometimes, but it can also make your mixes sound muddy as hell. Later on you may find yourself EQing the low end of your vocals out. Of course, I feel it depends on taste, if its your thang.... do what cha wanna do.... I cant tell you... who to rock it to! (a nice interpolation dont u think? lol) Keep this in mind and dont forget to solo your vocals every now and then! Peace out!