I can't tell you about large breed dogs and when to spay/heat cycles as I have a male.
But i do have one female rat terrier intact and believe me, it was not fun going thru a heat cycle. At least we are in the country so there aren't too many dogs in a small area to have to worry about visiting males. Just to be safe though, she went out always with Gunther, who acted as a bodyguard.
I also have an intact male rat terrier who we had to keep separted from her for obvious reasons. His whining was enough to drive me up the wall.
Also during her heat cycle she ate one of her panytliners which gave us a scare for a day or two until she finally started pooping again with the help of some advice from the vet.
I would totally recommend getting it done before a heat cycle. Less mess, less stress. Unfortunately for me, I promised the person I got them from to try and show my two buggars so I can't get them fixed, at least not yet. But believe me, if they don't perform in the ring or meet expectations, they will be on the operating table as fast as can be.