We've seen heroes sleeping and eating. They have have super-human abilities but they are still human. Characters are portrayed in different lights and we often get to see moments in their private lives. If we've learned anything from characters like Batman, it's that you have to be prepared before going into a battle. For the Dark Knight, he makes sure he knows everything about his opponents while also maintaining that his body and gadgets are fit for the task.
I'm sure this question has been raised before and I'll admit, I was a little hesitant about bringing it up but when do superheroes go to the bathroom? Prepping for a superhero battle might be sort of like getting ready to play in a sports game. I remember before football games in high school, you made sure you went to bathroom before hitting the field because there really wouldn't be a convenient time during the game. Do heroes make sure they visit the little boys' room before facing their opponents?
The only problem with this reasoning is heroes might not have the opportunity to go to the bathroom before fighting their enemies. Often, the battles can happen at a moments notice. These battles can lead to crazy chases or missions. Think about when the heroes were involved in their battles during Secret Invasion or Blackest Night. It's not like a hero can simply excuse themselves in the middle of a mission to answer the call of nature.
It's interesting that out of all the characters in comics, it's been Wolverine that has been shown going to the bathroom on several occasions. Shouldn't his healing factor somehow prevent him having to go to the bathroom so much? In New X-Men #142, he couldn't even drink a bottle of whiskey without having to visit the VIP urinals at the Hellfire Club.
What about other characters? Is it part of their superpowers to have optimal control over their bladder? I always said this was one of Jack Bauer's (from 24) superpowers. He would go 24 hours straight without having to relieve himself. If you're a speedster like Flash, obviously you could dash off the the bathroom in the middle of a fight. Other characters will just have to hope they can hold it until the fight is won.
But there is no predicting when you have to go to the bathroom. Sometimes it can just sneak up on you. After all, even Batman, the man who is always prepared, peed his pants.