Usually quality in pied will really be a matter of personal taste.
Pied are very variable and usually with your single gene pieds people will in my experience go for the medium to high white, they seems to be more desirable than low white but there are people liking low white as well, that generally affect pricing.
For combos it can be the same however when you start adding genes (depending on the gene) you can go either way (high or very low white), Black Pastel Pied are one of them, usually they are very high white, head pattern and maybe a little bit on the tail, I like them with pattern on the body, it makes them more attractive to ME, does not change their value but it's what I like to see when I open my enclosures.
To me any animal in my collection is about what I really want to see in my racks, not say I have this or that, just seeing something truly appealing to MY taste.