Will rounded corners fit in standard business card holders?

In almost every case, yes. Standard card sizes with rounded corners keep the same overall dimensions as square-cornered cards, so they slide into typical wallets, cardholders, and cases without trouble.

• Modern wallets, sleeves, and business-card cases handle rounded corners with no issue.
• The one exception is some older, tight-slot organizers, like traditional Rolodex-style holders, which may grip a rounded card less securely.
• If your audience still relies on that kind of storage, square corners are the safer choice.
• For everyone else, the rounded profile is purely a style decision.

Since the footprint does not change, compatibility is rarely a real concern for today's cardholders.