This is a personal decision, and one which always brings up differing opinions.
Just wanted to tell you our experience, our first girl Cassie was spayed at 6 months (collie cross). She died aged 15, we didn't ever have any issues with her.
Ruby (springer/retriever cross) is nearly 6, she was spayed at 6 months, recommended by a different vet. No problems as yet.
Jem has recently been spayed at 6 months.
You just have to do what you feel comfortable with