While a problem is being viewed, there are Display Options for both ViewEquations and ShowSavedAnswers.
ViewEquations is set globally in localOverrides.conf, can be revised in configuration of a course, and can be changed by a student.
It appears a student's default setting (in v2.7) for ShowSavedAnswers is NO for a closed assignment. It also appears that changing NO to YES during a session is not stored --- after a logout and login, the setting has reverted to NO.
A student might want NO while reworking old assignments during review for an exam. I think a more common revisiting of a problem will occur soon after an assignment closes when a student tries to understand (with aid of ShowCorrectAnswers and Solution) and complete a problem --- in that situation, viewing the most recent submitted (!) answer is apt to be helpful and should be enabled by default. [Note: this is not a request to retain and display old stuff entered using CheckAnswer.]
I suggest the default setting be YES for both open & closed assignments, I also suggest any change of that setting be retained between sessions.
Note: the setting for me as instructor defaults to YES and reverts to YES.
I suspect the bottom two items on the Permissions tab of Course Configuration are relevant.
Can show old answers by default (Student is default level)
Can always show old answers by default (Professor is default level)
Note: the "?" icon provides no help for that tab.
The reason this is set this way is exactly as you say "A student might want NO while reworking old assignments during review for an exam" and it is really pretty easy for a student to change this for one session. It is the way I as a prof would want it to work. But if you want it to work the other way, again as you noticed, simply set Can always show old answers by default to student level. Maybe toward the end of the course when students might be reviewing for the final, you could set it the other way. But either way, it's pretty easy for students to do what the want if they know they can switch the setting.