Just common sense of ergonomic UI, because I am no expert in this.
I do not like dialogs because they cover other UI elements and require
switching user attention.
If I need some input field I usually reserve a place on the screen with
confirmation and cancel buttons. I try to implement
checking-while-typing when possible and not too annoying.
A confirmation/commit button is IMO preferable, however modern UIs tend
to commit changes as soon as the UI element loses the focus.
For settings I use tabbed views. They tend to grow out of any measure.
Not a perfect solution, but IMO much better than dialogs and in any case
better than tree views when you need to expand and collapse nodes
looking for a setting parameter.
Modal dialogs are OK for emergencies when continuation is absolutely
impossible without user intervention.
One of few advantages of GTK is that widgets and containers
automatically expand and shrink. One should use this feature and avoid
fixed sizes. The minimum widget size, e.g. of a label or edit field can
be set.