If your "10% off" is ALWAYS going to exist, AND applies to all products in your store, that's fine.
However, all your prices will show up as regular prices, with no indication that they are supposedly saving 10%, until they go to checkout. If what you're after from a visual perspective is to show that things are 10% off, then you'll lose that edge.
Why not try this:
Regular customers: shop, seeing regular price plus sale of 10%
Teachers: shop, seeing regular price plus sale of 10%, and at checkout, get an additional 16.7% off the already-discounted prices, giving them a total of 25% off regular
Librarians: shop, regularly, but get an additional 22.22% off, giving a total of 30% off regular
.
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