I, too, would prefer to go to the more recent version if feasible (7.2 is expected to be out by the end of the year) - there are reasons why new versions are made, after all - but going from 5.6.30 to an earlier 5.6 would likely be less work.
you can compare Migration 5.6-7.0 with Migration 5.5-5.6 with what you have to see if anything jumps out at you as "oh yeah, I'm doing that". Note that the latter is for upgrading from 5.5 to 5.6, so probably aren't relevant to you (the change to GMP's model, for example, came in with 5.6.0), but the pages for the individual features mentioned give finer-grained version histories.
Ultimately, it comes down to a matter of suck it and see - which is why a test suite is useful. The number of times I've seen a place scared to leave 5.2 because they have no idea if something in their huge hairy site would break in an upgrade....