Doing something like converting input always to "exact" while mostly good behavior, could definititely be detrimental in other cases.
The general rule simply remains, while in the CAS avoid doing any approximations until any final step in an operation. That matches pretty well with what I was explained back in my school days from pretty much any professor too... 🙂
To quickly do this yourself, tap the history item once in the CAS (so the limit is selcted) and then press A B/C key. That throws "exact" around your expression for further evaluation.
Although I work for the HP calculator group as a head developer of the HP Prime, the views and opinions I post here are my own.