Trying to get that balance right is really hard, though. There's a certain amount of energy loss, depending on the ambient temperature, surface area, tank composition, and whether it is covered. Ideally, you'd have just enough watts to overcome heat loss. Then if the heaters stuck on, they wouldn't get too hot. Nice theory, and I can do the math to figure it out.
Then what happens is you lose power in the middle of the winter, put your tanks on a generator, there isn't enough excess capacity to keep the tank warm enough, and you lose fish that way. When we lost power a month ago, my 180 was running at 4-5 degrees under normal because the two 250W heaters couldn't keep up with the heat loss. You could tinker with it, just keep putting 50W heaters in until it was just right, and I'm sure with even more tinkering you could get them all calibrated the same.
There's also the angle that if one heater will fail on average every X number of hours, the more heaters you have, the more often at least one will fail. The lower wattage units are likely more reliable, but you're offsetting that by having more of them. Trying to figure out which one is stuck on could be a bit of fun, too.
Or you can spend $30, get a temperature controller and set it up to cut off 2 degrees above where you set your heaters, and be good to go.