I'm sorry, but the fearmongering around finasteride and dutasteride is unsubstantiated. Both have been studied to death, so far no study has been able to prove the so-called "post-finasteride syndrome". It's probably more of a psychological thing than anything due to reading other people fearmongering online.
The supposed DHT and it's metabolites' neurological protections have only been seen in an animal study. We know for a fact that the people who are born with 5-alpha reductase deficiency go through puberty too, testosterone alone provides normal increases in muscle mass and a normal libido. And there is no record of any significant neurological damage suffered by them due to never having DHT at all. There is also the fact that none of the ton of studies on fina has found anything significant on suppossed neurological damage either.
To answer the original, very reasonable, question: DHN is very hair friendly, using a 5-alpha blocker will prevent nandrolone from metabolizing into DHN, which means some other more androgenic compound might get your hair follicles.
I'd say that if your nandrolone dose is higher than your test dose, you should stop and clear duta before starting nandrolone. But if your test dose is going to be higher, it's probably worth it to block DHT. Eg, if your cycle is 300 test 600 deca, avoid duta. But if it is 600 test 300 deca, take it. This is just my personal opinion on the matter, I hope that makes sense.
The fact that you are taking RU-58841 makes this problem completely vanish, tho. AFAIK, RU is an anti-androgen that will prevent any androgen (be it DHN, DHT, nandrolone or any other) from adhering to your hair follicles. It is also local action (topical), so it will not block androgens working on the rest of your system, eg prostate. So no matter the androgens in your body, RU-58841 will prevent them from affecting your follicles.