It’s an easier decision with ER collet chucks, but either way I think of a 1/4” end mill as a 6.35mm end mill, just like I think of 3/4” plywood as 19.05mm plywood.
It’s not a crime to call it a 1/4” end mill, though. It is what it is.
To break down and start using fractions in your designs is not the same value call, though. It can get frustrating when it seems like the deck is stacked against you, but it’s not all that hard to find metric shop supplies if you exclude big box hardware stores.