testosterone converts to DHT via the 5 alpha reductase enzyme. DHT is far more androgenic than testosterone and is the main contributor to MPB in those genetically predisposed.
Finasteride is a 5 alpha reductase inhibitor, and therefore limits the conversion of testosterone to DHT via competitive inhibition.
Nandrolone itself is a 5 alpha reductase inhibitor (although it yields DHN in the process). So less DHT is produced if nandrolone is present. Also, DHN is much less androgenic than nandrolone and much less androgenic than DHT. So, finasteride with deca (nandrolone) should both work to reduce DHT levels, and since finasteride will inhibit 5a reductase, it'll also keep levels of DHN lower too.
The misconception that deca with finasteride is a bad combination stems from the the days when deca only cycles were more common. If one were running a deca only cycle (or any deca cycle without test), then finasteride would be a poor choice, because DHN is less androgenic than nandrolone. However, since we're assuming test to be included in a deca cycle, and both test and DHT are far more androgenic than nandrolone, if your goal is preventing the formation of the most androgenic compounds, then both deca and finasteride work together to reduce DHT formation.
To sum things up, if you're running a test + deca cycle and are worried about MPB (and you're susceptible), then including finasteride is a far better option than NOT including it, although the nandrolone will help to reduce DHT formation on its own to some extent.
This doesn't touch on the many positive effects of DHT (libido, muscle hardness, decreased SHBG and therefore increased bioavailable AAS).....this is only in the context of preventing hairloss.
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