I agree that since you've found them to be "good tenants", then its your judgement call regarding allowing this grace period. They may be notifying you ahead of time so that you don't think they are too irresponsible as tenants and will start Pay of Quit notice. However, I agree with others that you have to stick to the late fees (if your lease have it on). I have had a great tenant that notified me twice ~1-2 days ahead of rent due date (even though late fees are not charged til the 5th-7th of the month dependent of where the 5 days grace period ends on the week) to let me know that they will be late with the rent and that they will pay the late fees along with the rent.
One of my other tenant also has had late rents but never notified us, just that Innago will notify that the rent is withdrawn due to insufficient funds.
I find it it is better to keep easy (not calling/notifying every few days), clean tenants that will keep your place intact and never miss rents (may be late sometimes) than finding tenants that will keep your passive income investment not so passive.
Good luck in your decision.