OK, NOW I get it! LOL
OK, this is what I do. If I am using it as a base of a single tiered cake and need something to be able to lift and hold onto, then you should use a 10 inch for an 8 inch cake - every cake bought in bakeries, stores, etc, show it's on a plate that you can see.
However, if you are just using it for a stability issue for stacking cakes, buy for the size of pan. So, if you are stacking a 12 inch 9 inch and 6 inch cake, then use 12, 9 and 6 inch cake separators. When you are making a stacked cake, you apply borders later which will cover any evidence of there being any rim/gap to your tiered cake.
I think the lady who told you this thought you were just making a one layer cake where having a place to put your thumbs when carrying it is important.
does that make sense?