On a two-piece toilet, there are a few simple things that should be done during installation:
- the area around where the bolts go to hold the tank should be smoothed over; takes all of 10-seconds per hole. This allows the rubber washer to make a good seal*
- the nut that holds the flapper seat should be checked to verify it is tight, again, takes all of a few seconds.*
- make sure that the nut that holds the filler valve in place is tight
If you do that, and any conscientious installer should, you don't have problems.
* Two-piece only