If you do not change the rounding rules, Hours does not round your time automatically. If you want to change that, tap the gear button in the upper right, then Rounding rules.

Tap "Snap to" to change the increment that time blocks will snap to when you are editing time blocks. If you also want all time entries to snap to those increments on the fly as timers are running, turn the "Rounding" switch on.

When rounding is on, Hours will round your time off, meaning if a block is less than half way into the next increment, it will round down, and if it is more than half way, it will round up.