As has already been noted, .1 of an hour is six minutes, not five, therefore there is always a possibility of error in logging individual flights using this schema.
As to what's 'normal' - on my site the majority of english pilots log brakes-off to brakes-on in 5 minute increments, and log time elapsed in 5 minute increments. It's American pilots who tend to log decimals of hours for elapsed times - and most of them don't log brakes-off to brakes-on time at all.
As to the CAA's opinion. Well, one thing which prompted me to create the website was to improve my own accuracy of logging flights and summing the types of hours because I was worried about this. Since then loads of pilots have set up profiles, and many of them have quite large innacuracies in their data.
These are very experienced pilots, some of them professionals, many with 100s of hours, and they're logging P1s all over the place, they're logging dual time when they were P1, they're logging in command time when they were dual - all sorts of errors, causing some quite large discrepencies in their totals, and none of them has ever commented to me that the CAA ever even noticed.
I reckon that the examination they give to our treasured records is entirely cursory for most of the time - it's only when they're suspicious (or when they're researching accident causes), that they pay any real attention.....
Steve R