As a few others here have pointed out, all you are really loosing is the ability to hand out custom rewards of 1-4 XP, which the book never discusses (as well as always reminding players consulting printed resources that you've scaled XP down).
To be honest, aside from the aesthetic of larger numbers, I don't quite understand why Genesys uses 5 XP increments. I'm also not bothered by it. Just curious if this is a holdover from an earlier set of the rules. Your table, your rules.
It's minimal effort to change, so I'd say go ahead.