To start at the beginning - the progress bar does not have a setting in the menu to be turned on or off separately.
When you play back tracks, you see the WPS which is skinnable and the creators of the themes/WPSs are given the possibility to include a progress bar or not (if you want to know more, study the links above and the manual), a great percentage of WPSs/themes have one though.
If you want something similar like in mpegplayer that the progress bar just pops up if you fast-forward, rewind or pause, you probably have to code a WPS that does this yourself - or get someone else to do that for you. It should be possible with all the means Rockbox gives there (conditional viewports and playback states) but I haven't seen such a thing yet.