I round to the nearest 5 minutes. It's easy to put in my logbook. I keep an excel sheet as a copy which acts as back-up plus makes it so easy to add up, and to calculate things like "total multi, total tailwheel, latest twelve months aerobatics and total number of cross country trips" and other such combinations as wanted by insurance people and when applying for jobs. The decimal one kinda goes wrong if you don't have a Hobbs, since the tacho lies for circuits etc.