There are definitely situations where the perceived level will be louder between peaks being limited to -1dB or -0.1 db. It depends on how hard the music is hitting these levels and how often the music goes up there.
If the music is only hitting a peak level say 50 % of the time then yes you won't notice much of any difference between -1 and -0.1 dB. But there are other circumstances when there is a difference. Recently I mastered a hip hop album and these guys like it seriously loud. They like it literally slamming. I don't and I don't like mastering this hard but sometimes you just have to do it. At least PSP Xenon can handle it and keep the mix relatively together.
At first I set my upper limit to -1dB and the music was hitting that like all the time. Then I increased that to -0.1dB just to see if there was a difference and it was definitely louder and it was quite audible in fact. It is literally a dB higher so when the music is slamming that hard yes it makes a difference.
A lot of people around here are not mastering anywhere that hard so it is easy to make assertions that there is no difference between -1 db and -0.1 dB.
I have never had a master rejected either by a CD manufacturer when my masters are hitting -0.1 db but after reading Daniels's post I think it makes sense and I might pull that back now to -0.2 dB instead (on those very loud masters that is) Luckily most of my clients are not hitting that hard and I don't have to go there that often. I agree that limiting to -1dB is good for MP3 conversion. I actually do two masters one at -0.1db and a second at -1 dB for that purpose.