I cant really decide whether to mount my aio on the cpu after placing the mobo in the case or before. I think after is better but im not sure. (My case is cooler master mastercase h500 and the cooler is cooler master ml240l)
It depends on the case and what you are comfortable with. Some cases you have to do it before or after while others it doesn't matter. I typically get a feel for how I want the AIO to sit and then mount the board and all the other stuff and do the AIO almost last. You will want to watch out though, because if you need to install a back plate you will want to do that before you mount the board in the case.
I usually put the aio in after the mobo is in the case
Well, for the best CPU temps you should definitely mount the AIO at the front.
But as this Case has two massive 200mm blower style fan coolers at the front, just mount it a the top.