If your expensive monitor is freshly calibrated, you’ve profiled all of your printers, painted your room a neutral gray, and you wear nothing but nonreflective black clothing...well, you’re very stylish. But even the best monitor and most expensive desktop printer are, at best, an approximation of what will happen when your job goes to press. It’s important to have a realistic idea of how your job will look, and everything you can do to improve its appearance on your monitor and your in-house output is beneficial. You don’t want any surprises late in the game.
Consequently, your final judgment on job appearance should be based on contract proofs created by your print service provider.