Depends on the situation.
I just ordered an M&P 45, with the Apex kit installed right off the bat.
Now, I have fired an M&P before, and I know how I want a trigger to behave. The stock M&Ps just don't cut it. So, based on the advice of some experienced people, I'm getting the work done first thing.
If I was getting a gun that I didn't have any experience with, although it may have a reputation for a crummy trigger, I would want to get a feel for it first.