I have been looking up the stories of Roman history before I read them, helping improve my understanding. However it also spoils what I am reading. Should I continue this practice?
Do you have the impression that doing this helps you improve your Latin? If yes, then I don’t see why you shouldn’t do this.
In general I think using a translation or paraphrase in your native language to better understand the meaning of a Latin passage is a completely legitimate strategy and I don’t consider it “cheating”. However, an alternative strategy might be to switch to an easier text for a while in order to improve your Latin until you feel ready to read Roma Aeterna without any extra help.
Ørberg gives a summary in English of the storyline for each chapter in the Instructions accompanying Roma Aeterna. So you definitely shouldn’t feel like you’re cheating. 
I don’t think there’s such a thing as “cheating” when mustering learning strategies, only whether or not what you’re doing is conductive to your learning.
My answer probably depends on your level, which I don’t know. If you are having so much trouble that you can’t do it without that crutch, then maybe — but then again maybe you should be trying easier texts first. If on the other hand you’re just fine, and are just trying to get ahead faster, remember that there’s the twin danger of, one the one hand, failing to prepare yourself for other texts, which when read out of the blue won’t give you the benefit of telling beforehand what they are about, and on the other hand the problem of self-confirmation:
My grandmother told me once a story about a Latin exam about Caesar. Obviously Caesar would always win in De Bello Gallico, so she had no reason to suspect that this wasn’t the case. Of course, or else this wouldn’t be a story, in this particular case Caesar had lost the battle, and she failed the exam.