The definitive answer: Finasteride only has a negative effect while on deca, and only has a positive effect when on testosterone, or one of it's esters. It's as simple as that. It has no discernable effects, positive or negative, on any other steroid.
Deca and Finasteride are a bad combination. Testosterone is converted to DHT in the scalp by 5-a-reductase. DHT is MUCH worse on the hair, so inhibiting the enzyme (and reducing concentration of DHT) helps with hair loss. Deca on the other hand is converted to a much LESS harsh form by the enzyme. This is the main reason why Deca is so mild on the hairline. The hair follicles do not "see" as much Deca as they do the less toxic metabolite. Inhibit the enzyme in this case, however, and the more toxic parent compound (Nandrolone) predominates, and this increases hair toxicity.