I'm going to bring up cheating rumors to my friend before his wedding without also (or first) confronting his fiance
Do you plan to go to your best friend or his fiance first?
I prefer to talk to his fiance and let them figure out their shit
Its good to be truthful when dealing with your friends, especially if they are going to be married. Just be careful with how you bring this up. If you are too confrontational then you are probably going to do more harm than good.
I'm going to be straight forward because if I do it their way everything will not work out.
It is great that you want to go about this in an honest manner. You may be saving your best friend from getting into a marriage not worth being in by doing this.