Rounding to the nearest ten, and only the nearest ten, so the deciding digit is always the ones digit. Numbers run to three digits, which keeps the rounding place visible without the question becoming about which column to use.
This is where rounding usually starts, in second and third grade. Once the ones digit is a reliable signal - 5 through 9 rounds the tens digit up, 0 through 4 leaves it - the nearest 100 page moves the deciding digit one column left.
Look at the ones digit. If it is 5, 6, 7, 8 or 9, add one to the tens digit and make the ones a zero. If it is 0, 1, 2, 3 or 4, just make the ones a zero.
Up. 45 rounds to 50, 135 rounds to 140. That is the round-half-up rule, and it appears often enough on each sheet to practise.
Second and third grade, usually before rounding to larger places. Whole tens are easy to picture on a number line, which is why it comes first.
Turn on Include Answer Pages and a matching key prints behind each worksheet page, with the questions in the same order.