I didn't know if I should tell you or not.
First sentence refers to the past directly. "It was in the past that I did not know". Second sentence refers to the present. "I now don't know", but a second part of it implies that the subject was already told.
Generally you need to understand how the present perfect works to get that sentence. For example:
I wrote you the list. - I just say what happen in the past.
I have written you the list. - I am speaking about the present: it was in past that i have written but it is important that now you have the informations from the list.
So "I don't know if you should tell me?" would mean that I am still not told when "I don’t know if you should’ve told me" means I am was told already.
I am not native so I can be wrong though.