Not sure about the walls - I think a proper statistical analysis would be quite complicated.
On average, the Gold:Heart ratio of walls is almost identical to that of normal monsters, so I don't think there's too much wrong with just treating them as such, and destroying them anytime it helps you open up an area of the map that you want to explore. You just have to avoid situations where you're on 3 hearts and relying on a wall to give you the last 3 gold you need so that you can level up.
You could track which specific walls you've already destroyed, and figure out the probabilities for what's left, but that's too much work for me, and not necessary.
I suppose the ideal time to use them would be when you have 1-2 hearts left, and need 3 gold to level up. Spending those hearts on killing monsters doesn't really help you, but there's a chance that a wall will break in 1-2 hits and give you the 3 gold you need, to save you wasting a healing scroll.
I tend to mostly use them when I'm about to heal or level up (and already have enough gold), but have an awkward number of hearts left, that I don't want to waste - particulaly very early in the game (when there might not be many rats about), and very late in the game, as you're mopping up the final few monsters, and burning through those healing scrolls.