The first time launching Source/Sort Members Eclipse asks whether sort fields/members as well and allows to make "Do not ask again ..." choice.
I've checked "Do not ask again ..." combo and as as the result Eclipse does not ask me any more.
The question is: how to reset this option to initial state, thus making Eclipse to ask me every time?
In Eclipse, go to Preferences and click on the main Java tree item. It will say "General settings for Java development"
Near the bottom there will be a section called Java dialogs. Click the clear button and you will start seeing this dialog again. The only problem is that all dialogs in the JDT that you clicked "Do not ask again" will show up again, but this will solve your problem.