If you do not change the rounding rules, Hours does not round your time automatically. If you want to change that, click the gear button (Preferences) in the sidebar, then you'll see Rounding rules as the fourth option down from the top.
Click the Rounding toggle to turn rounding on. Then, click the "Snap to" dropdown to change the increment that time blocks will snap to when you are editing time blocks, and all time entries will snap to those increments on the fly as timers are running.
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.