I log everything to the nearest 5 minutes and it has always been acceptable to the CAA. On some flights you lose a minute or two and on some you gain, "swings and roundabouts" as they say.
Datcons tend to count in decimals, so
0.1 = 5 minutes
0.2 = 10 minutes
0.3 = 20 minutes
0.4 = 25 minutes
0.5 = 30 minutes
0.6 = 35 minutes
0.7 = 40 minutes
0.8 = 50 minutes
0.9 = 55 minutes
1.0 = 60 minutes