Don't mow with both the weights and the FEL -- not necessary and the front tires will dig into the ground when turning leaving ruts. I found that you need some weight up front with the MX6 or you will have an extremely light front end (turning is a problem).
Since you have an MX6, I assume that you are cutting some pretty rough stuff. I find that the FEL is really handy to have on. It has helped pull me out of wet areas that would have been impossible in any other case. I also find that if you have both the FEL and the MX6 on and you are cutting over bumpy areas, you should be careful not to stress the front tie rods -- I have broken two tie rods when encountering holes in long grassy areas.
To protect myself from this, I drive with the FEL quite low to the ground (4" or so) and level. If I encounter a large indentation on the ground (in which the front tire sinks), the FEL ends up skimming along the ground and limits how far the tire sinks (protecting the tie-rod).
I have quite bumpy fields as I have a landscaping tree farm. The holes caused by digging the trees are filled immediately, but after a winter, there is normally a 6" indentation after settling.