I would have her spayed halfway between seasons, i.e. three months after she was last in. I have had bitches done at 6 months and various other ages without any problems.
I can understand what jules says about coats, but with a short coated breed there is not a problem, and it does make life a lot easier, without having to worry about unwanted litters, pyometra or mammary tumours later in life. Just watch her weight, as being a Lab she will no doubt eat for England anyway, and provided you keep her fit and active she should not put on any unnecessary weight.