Sealing before grouting is essential to ensure pores are filled.
Regardless of how experienced an individual is when it comes to grouting polished natural stone it may stain when grouting. There may also be a 'grout film residue', this can be a slight film or deposits of grout.
There are lots of stone sealers available in Australia. Whichever you prefer please follow the directions for sealing. Most require 1 coat before sealing and 1 coat after.