Short answer:
I generally avoid anyone with WE under 10.
I think hard before signing guys with WE between 10 and 20.
I have no issue with anyone with WE over 20.
Long answer:
It really depends on the player. An A+ prestige school in my D1 conference recently signed a guy with a 1 WE.
A guy with WE that low will hardly improve at all over his career, but in this case the guy had starting ratings high enough (all cores in the 80s and 90s) to justify signing him anyway.
Basically, WE (along with potential) affects how quickly a guy improves. I have found that anyone who starts over 20 usually doesn't have trouble reaching his potential in core ratings.
Guys under 20 may not ever fully reach their potential, but with playing time, you can improve their WE, and they usually grow pretty well. If a guy has great starting ratings and a WE in the teens, I'll sign him.
Guys under 10, however, are another story. The WE is so low it is very hard to improve, even with lots of playing time. They are just never going to improve much. So, the only way I'd sign one is if their starting ratings were fantastic. This happens in D1 sometimes, but almost never in D3.