Playing up a year but still with in the bracket is generally casually accepted in the top tier levels. It seems that in Tier III and Tier II (A and AA) parents can be a bit more strange about it. Parents tend to mellow a bit once their player is AAA. The players, on the other hand, have no problems with it provided the child playing up has the skill set and conficence level to be there.
While much can be said for playing up a year, it is not a good idea to play outside ones age bracket. If a Mite, Squirt or Bantam play inside that bracket. Many associations will not allow a player to move up outside their age bracket and I agree with the policy.
A child needs his/her time to be a super-star. Confidence is solidified and it helps those playing with these exceptional kids to also improve. Obviously, each individual case needs to be looked at to make the right decision. If the child has been a top player for a couple seasons, its good to move them up. If its a first for your child to be a dominate player you might consider holding back and letting them shine in the spotlight a bit.