Is it common for a molar to not get a temporary crown after a root canal? is shaving down the tooth enough to prevent damage even though i was told that i need to chew on the side of my mouth with the new root canal given the other side had a surgery?

It is neither common or uncommon. Each dentist makes their own decision regarding fabrication of temporary prosthetics.

After the root canal your tooth is brittle and can fracture if a crown is not constructed. A temporary crown is not necessarily needed if you are getting a cap[crown] in the very near future. The treating dentist will determine what is best. If the tooth has a large opening than some type of build is necessary.