That is why I always save my free change and skip for the last tile, except when the first tile is Destiny. If it is a Destiny or Transport I first change and if it becomes a Destiny or Transport then skip. I also change when my score going into the last tile is as follows (x means 5, 6, or 7) to see if I can get something that will take me to the next level after Guardian (a transformation and then a double makes the Guardian 100 points):

Between x00 and x29 (both inclusive) unless the last tile is Transformation
Between x30 and x39 (both inclusive) unless the last tile is Transformation or Questions, or Monter attack (better odds on getting a pit)
Between x40 and x44 (both inclusive) if last tile is Shuffle, Discount (unless you are prepared to spend gems) or Shuffle that only give you 5 points.

The above is for the Easy Lab. Adjust for the number of questions on Normal (3) and Hard (4).