It's possible to use kiosk printing to print receipts without a dialog window from Chrome. We should add a tip about it to either the manual or the wiki. See top answer here: https://stackoverflow.com/questions/20579959/silent-printing-direct-using-kiosk-mode-in-google-chrome
Possibly also worth documenting QZTray as a direct print solution alternative here. It allows for direct print in a different way, instead of passing chrome a flag and being tied to the default printer, QZ requires a tray applet installed on your machine that Koha can communicate with directly for printing. This allows skipping the print dialogs as well as adding support for cash registers and things.