How does one calculate a tenth of an hour?

My understanding is that because chunked minutes as unit-based billing is binary (it's either earned or it isn't), then it's either rounded up or down as the case may be.

In other words, 00:00 (mm:ss) to 03:00 would not be billable because you've worked for less than half a unit of time. However, 03:01 to 6:00 would be billable, because you've worked more than half the unit of time. This is why one can "punch out", as it were, at 05:27:01 (hh:mm:ss) p.m. and bill as though you punched out at 05:30:00 p.m., because 27:01 is more than half of the period from 24:00 to 30:00. Where as if you actually leave at 5:26:59 seconds, technically your time has to reflect that you worked until 5:24

Now I'm hearing that people are billing 1 minute as a tenth of an hour, based on the theory that so long as one second has be earned, the whole 360 seconds (6 minutes) has been earned.

This is theoretically - I suppose I'm looking for the correct "default" rule, so assume not specified in retainer or firm policy.