It would absolutely be good enough if the tools remember their size for only the current session.
Next time you run Paint.NET, as you say, you would probably want a different size on your tools,
so they can still have the preset value, size 2.
Or maybe an option to set your own preset values would be suitable, so that your Rubber isn't size 2
when you start. It would be nice, even though it's not so important.
The important thing is to reduce the number of times you need to change the size of your tools,
so it's only done when it's really needed.
If you paint, you mostly work with small pencils, ranging from maybe size 1-7 and when erasing you seldom
need a smaller rubber then size 10. With this feature in the program, when sizes of the tools is set to your
liking; you only change the size when you need a smaller pencil/brush and not because you just need to use
another tool with a different size for a short time, before heading back to your pencil/brush.