Billing small time increments

It's generally a good idea to get clients onto a monthly retainer

This will help your business cashflow and gives your client some support hours that you can plan your work around.

That's not always possible for each client/project though and sometimes changes take longer than the hours inside their retainer agreement.

Recently our client launched their site and we had a small change to make to it yesterday. It literally took less than 5 minutes to make the change and I decided not to bill.

That said, time is time - and all of those minutes soon add up.

What's your approach to stuff like this? Bill a minimum of an hour or a percentage of your hourly rate?