Yea do em all at once if they are all ready, but no if not. I compare it to tomatoes in the garden. Just because ones red doesnt mean ya take em all, but if they are all red why not?
I got my record harvest with a double harvest. I aim for flat top and limit the undergrowth, one time she bushed up thicker than i planned. When i mowed top off there was a lot more undergrowth than what i thought. I let em go, kept a scope on em, they ended up going full nother month! Literally doubled my harvest, as in quantity. However the second round was good, but not great appearance as density never became what tops were. I dont think ill aim to take it to that extreme again though...
That being said take em when ready whatever that means to your grow style...