I'm pretty certain you will be fine. I have never really worried about point releases as long as the "branch" version remained the same. I think you will find that 99.9%, if not 100%, of the things you are working through in why's guide will still work no matter which version of Ruby 1.8 you use.
Here is a snippet from the Ruby 1.8 branch policy from the ruby-lang issue tracker site:
The 1.8 branch, or in general, a
"stable" branch is developed and
maintained for most casual use by
average ruby programmers. Besides
fixes for security problems and
run-time bugs, there can be
performance improvements, feature
enhancements and library updates made
on the branch. Those kinds of
aggressive changes may occur, however,
only on condition that backward
compatibilities and run-time
stabilities are retained to a high
degree.
...
Committers are allowed to make a
commit on a stable branch without
explicit approval from the branch
manager, as long as it does not break
any backward compatibilities, that is,
it does not delete or change any
existing features.
A list of exceptions is as follows. A
commit may break backward
compatibilities only when all of the
following conditions are met:
- It only affects: 1) behaviors which
contradicts with the document,
2) internal, unpublicized interface, 3)
undocumented features, 4) undefined
behaviors, or 5) presumable misuses.
- The new behavior will not change again
in nor contradict with the next major
version series of Ruby.
- If it is
assumed that it may silently affect
existing code in a subtle way,
sufficient information and/or a
support tool must be supplied with to
help users fix their existing code.
- It is well considered and reviewed in
public and approved by the branch
manager.
- It is well documented and
announced in ChangeLog and NEWS as
soon as it goes in.