I cannot walk with a full bowl of water without spilling it.  I always change out the water bowls and if an owner fills the bowl to the top and their dog or cat hasn't had very much of it, I have a very hard time getting to the sink to dump it out and refill it.  When I refill the bowl, I never fill it all the way to the top.  Something I've just noticed over time.  (As a pet sitter, I have a lot of time to think..... can you tell??? :) )