The last time Rosie had blood work done her WBC came back low too. She had an infection at the time and they said she was "stressed". But they felt the stress was from the illness. I don't think anything else came back abnormal.
If you trust your vet than I would go with what he tells you. I completely trust mine and feel that if he was questioning anything he would send me to a specialist. As I am sure yours would. They did make me repeat the bloodwork I think two weeks or so later. You might ask about that.
Oh, and it is ok for us to be worry warts!
That is what we Yorkie momma's do!