It is important that students understand what “rounding” means, not just how to do it. Rounding refers to finding the nearest “round” number. (Round numbers end in at least one zero.) When you round a number, there are always two possibilities - the next highest round number, and the next lowest round number.
To illustrate this idea, a number line comes in handy. “Round 3,586 to the nearest ten.”
Tens usually end in one zero. So at least one of the nearest round numbers will have only one zero.
This way, students can see that 3,589 is closer to 3,590.
Of course, most students know the shortcut. I allow my students to use this method only if they can explain why it works. Here it is:
“Round 3,586 to the nearest ten.”
Write the number.
Underline the digit in the tens place.
Use the neighbor digit to the right to tell you to round up or down.