I would use the CRP-5 for most things, as it is quick and self-explanitory, with less to remember and lots of reminders printed on it. In particular RAS/TAS/Mach and indicated/true altitude it is the only sensible way. The CRP-5 is certainly the best instrument of its type (as well as being the one used by JAA to work out the answers). Ignore instructors who tell you to throw it away, they are probably not very confident about it.
There are polar stereographic questions in the question bank. Whether they come up or not I cannot comment.
Best of luck!
Send Clowns
Gen Nav instructor
BCFT