Using org-mode 5.23a with emacs 22.1, I still don't see timestamprounding working during clock in/clock out (OR shift up/down on the timestamp).
Also, I'm slightly confused by an earlier thread we had on this subject. At that time I was using 5.20 (or 5.21, I don't remember) and it was mentioned that it would not be possible to do  rounding when inserting time. However the custom options for 5.23a show that there is a variable for rounding during time insertion as well as during time modification via the up/down key - my .emacs setting for this looks as follows...
 '(org-time-stamp-rounding-minutes (quote (5 5)))

However neither seem to have an effect...
Hmmm.... 

Jose