When you are setting an event in your calendar, chances are that the nearest rounded-off number will do. But the calendar app in iOS 7 gives you only single-digit intervals by default in the 'minutes' column. Fortunately, you can change that to the old 5-minute intervals of iOS 6 with a double-tap.
In the Calendar app, when you are setting the time for an event, double-tap the minutes column. This works for both the Start and End times. The double-tap will change the intervals to 5-minute lengths, and you can double-tap again to change it back to 1-minute intervals.
To fix other iOS 7 annoyances, check out our comprehensive guide.