Every customer has different needs. Some want to follow the portrait through every step of the drawing process. This type of client wants previews and updates all the way through the job and often.
problem with the in-progress, unfinished preview from the artist's
point of view is that the artwork does not yet look great. Because of
this, an unfinished picture often causes too many problems in that it
causes the client to doubt their decision to hire you. They may
nitpick the artwork.
Of course, the artwork's not perfect at this point, because it’s not finished!
negative aspect is that if you are sensitive, the criticism may effect
your ability to create. Your self esteem may suffer.
I think it's best to avoid any in-progress previews, but rough sketches of your layout are okay.
can you avoid sending a client an unfinished preview? You can
diplomatically avoid sending an unfinished picture by letting the client
know what to expect ahead of time, when you accept the commission.
You can tell the client you will send a preview by email when the art is finished, or you can tell them that you will send one in-progress picture. Assure them that you will stay in touch with them throughout the process.