In my experience, the bigger a cosplay is the more likely someone will want to take a photo with it. For example, at Dragon*Con last year there were a few life sized Totoros, I'm talking 1 to 1 ratio and it was fantastic. There were lines of people waiting to get their picture with them because for something like that it is far more interesting to be in the moment with the character.
Use your judgement, if the person seems busy just snap a quick shot and be on your way. If they look like they have time to spare, ask if you can get their picture with them. This gives them the opportunity to accept or deny.
On the flip side if someone asks to get there picture with you, be sure YOU are comfortable with it. Don't ever feel pressured into doing something that you do not want to do.
All in all just be respectful and use good judgement, I'm 99% sure everything will go amazingly!