is it bad for the fan to run?

[simplechamp]

Well, if it's turning on and off multiple times per run then that means you are hovering right around the 150F point, which I wouldn't be too concerned about. But if it is coming on early in the run and staying on until the end you might be pushing the temps a bit. You don't want to rely on the fan, you want it to be setup so if the fan failed your ESC would get hot, but not enough to thermal.

You probably should gear down so you are geared for the same speed with 5s as you were when using 4s. This will help reduce your temps.