You’re completely right at the beginning – caching is always a good idea, when you deal with results that are hard to get. But…
That’s not the case in here. Registering post types and meta boxes is not complex at all – it’s just few PHP operations. They won’t take much time (nor computation power) to be done.
So what is complex? Everything that takes long time, needs many DB queries or a lot of computation power/resources.
Let’s say you want to calculate number of words on your site. So you have to loop through all posts and count words. That’s perfect case for caching – it’s complex, because it’ll take much time – so you do it once and store the result in cache (probably in transients) for later.