Hi. By the tone, or words, in your question, you are in a nice state of mind. I have been schooled by landlords who have done it for years, while we are not always blessed to have good tenanats, sometimes they do have different life situations that you may want to be considerate of. Life situations like a tenant may discover they have cancer; well you may chose to give them a break. You should also consider your situation, e.g. you live in a city where you have no problem finding renters, vs. you live in a city where it is a small community and renters are scarce and low income.
The rule of thumb under normal circumstances is you have provided the due date and the grace period in the contract. Once the late date has passed (the 5th) you send a late notice with the amount due plus the late fee. It seems cold, but you need to stick with your contract so they know you are serious and your bill has the utmost priority to be paid before they pay anyone else. Stick to your contract. don't start sending extra notices to be nice. Also if you feel like you are two nice, find a reputable property managment company and let them deal with it. Good Luck.