I start my puppies young and encourage sitting from the time they are 6 weeks old. They get treats when they sit. The above picture is Lyric's litter around 6-7 weeks old(Echo, Darby,Tanner, Becca and Bryn). They all learned to sit quite willingly and readily.
In the video below Echo is about 9.5 weeks. She has learned to sit but is being reinforced for this behavior with a clicker. The video shows sitting, downing, touch and leave it.
One thing I never ask my puppies to do is to sit on hard surfaces until they are rock solid with their sits. I think it must hurt their hocks and rears and have found they will often avoid sitting on concrete or asphalt if possible.
Occationally in the show ring the puppy gets confused and will sit instead of stand for the judge. So what! I tell the judge the puppy must be confused about which ring it's in and the judges are quite impressed. They even tell me "but a greyhound can't sit".
A good solid sit is important for obedience as well as for photo taking.
Echo has the most rock solid steady sit of any dog. In fact I followed her thoughout the day and caught her sitting several times just because.
A good sit is useful for so many things so get out and train your hounds!!