This came up in similar threads when I was reading something more current, and I always find it a really interesting conversation. Apologies if I'm not responding to the most recent conversation on this, since thread topics tend to repeat themselves over time.
I believe that I should get top-of-the-line service from a top-of-the-line store no matter what I am wearing. However, in reality, that is not the case. I find I have a hard time getting waited on in many stores, so I prefer to take a little extra effort with my appearance to save the time I'd otherwise spend trying to get served and avoid feeling crappy because of snooty salespeople.
If I am shopping for clothing, I do tend to wear nice clothes but wear my usual hair and makeup to make sure clothes will look good on me the way I generally intend to wear them. If I am shopping for handbags, I like to get dressed up to see how much more the purse can add to my outfit, since a nice purse cannot save a bad outfit. In fact, if there's a particular bag I have my eye on, I'll wear an outfit I think I'd wear with that bag to see if it's as good a match for me as I think. So there is some practical consideration there, too.