I kinda regard it as being invasive, in as far as I'll lose all my customised settings by doing that.
Be that as it may, however, I am actually posting here again to inform the community (perhaps I'm stating the obvious though) that I've since discovered that Premiere Pro's behaviour in this aspect appears dependent on the video frame rate being worked with.
My original post followed my discovery of this issue when editing mp4 files that came from screen recording software, captured at what was apparently 10fps. However, today when I was editing "proper" video footage shot on a DSLR at 25fps, the Step Forward 1 Frame / Step Back 1 Frame options worked perfectly - advancing / retiring the video by one frame per click as they should (without me having changed any settings between the two editing exercises).
My conclusion is therefore that working at too slow a frame rate probably doesn't offer Premiere Pro enough "granularity" to be able to work with individual frames, in the way that it can when there are more frames to use per second. Probably obvious now, in retrospect!