Better to be overdressed than underdressed. I always bring 2 suits, and 2 shirts (plus backups, but that's a different story). I always have a dinner jacket/tux pants and a regular black suit depending on the level of formalness. I also have a white shirt and black shirt. This way between the combination of jackets and shirts, I cover a wide range of dress codes. Granted, I tend to perform at very formal weddings, so I'm almost always in a dinner jacket/white shirt combo, but my sweets are a little less formal... although girls are in dresses and guys are at minimum in khakis and button down... so for those I wear a regular suit with just a black shirt, no tie.
And yes... please tailor your suits. Don't have to buy custom made... but with how cheap they are relative to an off the rack suit, it doesn't make sense not to.