In every case, it is always best to reach your goal weight first, if at all possible before having a tummy tuck. At the very least, get down to a weight that you know you can maintain easily. This will only make things better for you, as you will reduce the amount of revisions that may be necessary. If you lose weight after everything is tightened then things might loosen a bit more, creating the possible need for additional surgery.
If you are actively trying to lose weight still, then keep up the good work until you are at your goal or very close to it, unless you have lost a significant amount and are experiencing itching and irritation because of the loose folds of skin hanging down.
In that case go ahead with the procedure, because there will always be SOME loosening of the skin post-op regardless, and there is no need to continue suffering needlessly. Also, if you have reached a stable weight and have stayed there for several months, it should also be ok to proceed with your tummy tuck without too much potential for loosening later.