The reason why they recommend running those tanks at the wattages they do is because with the lower resistance it takes more wattage to heat up the thicker wire to work properly....and most people don't like running them at lower wattages because at those settings they don't get very hot or produce much vapor..
Higher resistance coils are usually made with thinner wire that has more resistance allowing them to heat up quicker at lower wattages but because the wire is thinner you have less surface area producing less flavor and vapor....the difference between the two is kinda like the difference between a 6 lane freeway and a two lane highway.
The sub ohm coils are like the six lane freeway, it takes a lot of cars jamming into it to slow traffic down and back things up (sub ohm coils with thicker, lower resistance wire needing more watts to heat up because it's got less resistance slowing down the electrical current running threw it.)
Where as plus ohm coils are like two lane country highways where it only takes a handful of cars to cause a backup (plus ohm coils with thinner, higher resistance wire needing less watts to heat up because the small wire creates a smaller choke point for the current running threw it)
The coils we use run off of ohm's law but another way to look at is is the "friction creates heat' principle. The smaller the wire/coil the less room for the electrical current running threw it has room to move and friction is created by the current passing threw it causing it to heat up. The larger the wire used in the coil the more room the current has to move threw it the less friction created as it travels threw it and the coil won't get nearly as hot with out running more current threw it.
That's why most .5 coils recommend 20+ plus watts, to put enough current threw them to heat up the coil properly.. That all being said...if you like the vape that a .5 ohm coil puts off at 12 watts than vape it at 12 watts....you can't hurt a coil buy running it at lower wattages. You can cause dry hits and burn wicks by cranking your mod up too high for the coil to handle but not too low...the worst thing that will happen is some of the liquid won't fully vaporize and you might get a bit in your mouth, unpleasant but harmless. Also, if you like running your mod below 20 watts some sub ohm tanks have higher resistance coils, like the 1.2 ohm coil for the Kanger Subtanks, that are meant to be ran between 10-18 watts. You can always look up and try one of those or the equivalent for your tank. But if you like what your .5 ohm coil puts out at 12 watts or something like that than vape on....vaping is a very personal thing all about personal preferences....I can tell you what I like and how I like to vape it all day long, doesn't mean you'll ever like the same thing. It's all about what works for you.
Happy Vaping!