Jepp ATC pg AU-714 6.1.2 "The pilot of an IFR flight departing from a non-towered aerodrome must report 'IFR' when making first contact with ATS."
As NZ said, sometimes people use this as a starting point to give ATC a chance to pick up their pen before you go into the barrage of information. You would be surprised how much other stuff the guy on the console has to do apart from talk to you. Plenty of times they will tell you to 'standby' while attending to another matter before your taxi call - especially when things are busy. Put yourself in the other bloke's shoes for a moment and see what you would prefer!