I'm cool with CFIs charging ground time, but as a suggestion make it clear to the student when the clock actually starts ticking and when it ends.

I've had CFIs in the past charge me for ground time when we were clearly "shooting the bull". as stated above, I get you would start charging because you are making yourself available. But when we are hanging around because the plane is late or not immediately available or waiting for bad weather to clear up, that's not alright to charge a student for that stuff.

My two cents