It's like the Price is Right, you try to get as close as possible without going over.
Actually its not that simple. Sure you try to get as close as possible, but only by using stuff that offers penetration at good value.
Many people that do not understands this critical distinction go full penetration from all sources possible
Anyway, regarding DPS tests you dont go for full penetration anyway. You for a value that when added with common debuffs is ~18k. You do that by simply checking some builds, most of them are aiming at optimal penetration value of solo player (without any debuffs)