It's all very individual how much AI you might need. In my opinion lowish TRT doses like 100 to 125 mg/wk you probably wouldn't need any. Highish doses like 150 to 200 mg/wk you probably would.
Now you are talking about adding nandrolone in a dose sufficient to grow some muscle. It's debatable, but I'd say that's a minimum of 400 mg/wk. Nandrolone aromatizes at about 1/4 to 1/3 as much as test, so let's say your test dose was 125 mg/wk and you added in 400 mg/wk nandrolone which aromatizes equivalent to 100 mg/wk test. Your total aromatizing load would be 225 mg/wk equivalent, so I'd say you'd need an AI at that level.
In other words, to answer your question, I think if you get into the muscle building range you would require an AI.
For labs, you would want to watch E2 and prolactin. If you are aggresive at managing your E2 with an AI I doubt you would need anything for prolactin at 400 mg/wk - provided you didn't run it too long. If you went without AI, not recommended, you would want to run some caber or prami to keep prolactin in check.
If by chance you let both E2 and prolactin run away, you'd find out what deca dick means.