Well, look at it this way. You're cycling highly anabolic and androgenic steroids for a specific period of time, increasing the levels of DHT in your blood for roughly that same period of time.
DHT causes hair loss by activating the miniaturization of the hair. Some believe this occurs because DHT limits oxygen and nutrients (blood flow) to the follicle.
Baldness is caused by progressive miniaturization. I.e., the DHT is always present, and hair will fall out due to natural growth cycles, but the next hair that grows in will be a little thinner due to the lack of blood flow to the follicle, and this process will repeat until you go bald.
If you're taking steroids, you're only dealing with too much DHT while you're on the cycle. When you stop, your levels will return to normal and your hair will act as it is genetically programmed to act.
So, for cycles that are a reasonable length, I don't think finasteride is necessary.
All steroids do is introduce more DHT into the body to miniaturize hair faster than it normally would. But that is ONLY if you're genetically prone to hair loss. If you back off, the DHT levels drop and your hair follicle is able to get the necessary nutrients once again.
Some guys can take anything they want and not worry about hair loss at all.