When I started playing (late 80s), pentatonics were how people were introduced to soloing. There is alot you can do with pentatonics and there are fewer notes to contend with, making them perfect for beginners.
Most pentatonic patterns are 2NPS.
If you start any pentatonic lick with a downstroke (like most people do), you will not have any string collisions if you have a downward pickslant. If you want to play pentatonics with a primary upslant, you will need to start every pentatonic phrase on an upstroke.
I wonder if there is a correlation with primary pickslant and ability to play pentatonics? If a young player is never introduced to pentatonics and blues licks, he might start out with upslanting, in which case pentatonics might feel alien. (Unless he also trains to start everything with an upstroke.)
For this reason alone, should primary downward pickslanting be favored? There is a whole blues lexicon that generally favors downslanting, while everything else can be done with either. Additionally, most metal rythym is primarily downstrokes, so starting phrases with a downstroke is second nature.
If someone starts with 3nps (modal) patterns, the first string switch benefits an upslant. Which might explain why people are primary upslant players.
IOW, if you are teaching a young student to pick, should he be encouraged to adopt a mild downward slant to avoid potential “dexterity” issues later on?