It depends on the type of person you are and to what extent resale value concerns you.
I'll admit, I have a problem, as I clean my gadgets every day. I once sold a fully loaded 17" MBP because the rubber feet attached to the bottom case showed wear.
At the time I didn't know you could simply purchase a replacement bottom case cheaply.
To give you a better idea of my brain's messed up state, ever since that time, I carry a microfibre cloth and also have a few at home
My MacBooks only get placed on top of these when on a hard surface. I never drag the laptop across a surface either to prevent wear marks.
I also clean the keyboard/trackpad after any prolonged use to remove any oil marks. The ports and all surfaces are also cleaned daily.
Now you may be wondering why I'm posting on a forum instead of spending time with a psychologist.
I tend to upgrade at least once per year. Normally I describe the clean/as new condition of the laptops in my eBay sales.
This can sometimes work in my favour as the last few MBPs I sold were purchased by what I assume to be buyers who cared about the aesthetic condition of the unit (most recent sold for ~AU$300 less than what I paid for it including fees).