I have my first Officer reporting every time we switch controllers that we are IFR. This is not necessary since controllers have your IFR flight plan. They just need to verify your call sign and position. I think PF3 should eliminate this. Just say something like "Miami Center, JetBlue 311 with you Flight Level 330" or if they don't have radar service, "Port Au Prince Center, Jet Blue 311 approaching RETAK FL 310 1622, TUMAR 1628, ALBEE next" but you never end these calls saying "I F R". The opposite is true if you are VFR you should say it.