I don't think that there will be too much trouble, as long as you follow every changelog tip (9.0->9.1, 9.1->9.2 and 9.2->9.3)
What if we don't follow the changelog tip? In this case, we have only the 9.0 and 9.3 binaries installed and pg_upgrade directly to 9.3. Does that cause fear?
Not so much fear as concern. One of my rules is that the good will take care of itself it is the bad you have to plan for. In this case it is not the nine changes that do not impact your code but the one that does. When going through the change logs the incompatibilities are listed at the top under the Migration to Version 9.* header. It does not take to long to double check. If you want to save time just look at the major version changes, i.e 9.X versus 9.X.x as incompatibilities are allowed in major version changes. Not that they do not happen in minor releases, but that is generally done to fix a security/major bug and I do not recall any in the range you are looking at.