I always explain at the outset that if I sit down and do something I would not be doing if you were not paying me, I am going to charge you.
I am not blessed with infinite time. I am not doing this for you because I'm a nice guy. I'm doing the work because you are paying me. That is the entire deal.
And if there's some compelling reason not to charge, for the love of god, make sure the hours show up on the bill, with a seperate row for credited hours.
You are doing your client a huge disservice by hiding hours you actually worked, even if you don't charge for them.
Hiding hours gives a wrong impression of how long a certain job will take and how much work is really involved. Same situation I see in large organisations and developers working long hours to get a project done because they've under-quoted. The employer then uses those metrics to measure future projects. Then when the employee's eventually burns out or start a family and transition into normal working hours the employer will see a sudden decline in productivity and assume they've been slacking off.
In the end you have the best intentions for your client but can end up making it worse for yourself going forward. Quote down to the minute you've been working on a project, if the client is un-happy then that is something you can work with.
I am not blessed with infinite time. I am not doing this for you because I'm a nice guy. I'm doing the work because you are paying me. That is the entire deal.
If you are on the fence watch this: https://creativemornings.com/talks/mike-monteiro--2/1