It is all right to spy on an ally. I feel that it is all right because they are doing the same thing on the other side. You are supposed to be able to trust your allies but they have been known to catch you off guard and attack if they have to.
Spying on an ally is often a tactic of the jealous or insecure, but it can provide useful tools or information that can either assuage or strike fear in the spying party. Careful measures should be taken to prevent leaks revealing the ongoing spying, because evidence of spying could severly hurt the relationship and invite even more trust issues.
I believe in certain circumstances it is ok to spy on ally, I believe a Nation needs to be certain that their ally is not actively trying to plot against them or is planning to do something harmful to the citizens of their nation, I feel it is justifiable to spy on an ally.
I think it is okay for countries to spy on an ally. As long as no laws are being broken, I do not see anything wrong with countries spying on allies. I do think that in terms of legality, it might be a bit difficult though. A country cannot completely trust another.
No, you do not need to be spying on a person, or country, that you think is your ally. I think that it is a good thing to spy on your enemy, but if you are spying on your ally and they find out it will end up real bad.