There is a possible workaround. Download S&T Keys free addon (see link in my signature) for the S&T year version you are using.
Install it and use it while navigating with Streets & Trips. When navigating in fullscreen, you can do this tip(extracted from S&T Keys online manual):
Tips: when doing gps navigation in fullscreen mode, the bottom guidance pane (where it is written the next street direction) can be completely removed from screen, actually driving with a map covering your entire screen. To remove the bottom guidance pane, just press "O" hotkey.
You can put back the bottom guidance pane as often as you want using the same trick