Yes, but there is subtle distinction that you are missing.
3.5 does not round to 4.0–it rounds to 4. When you round, you are eliminating significant figures. Keeping the same number of significant figures is not rounding. It is changing the number to a different number. You see, 3.5 and 4 represent the same number. They just have a different number of significant figures. On the other hand, 3.5 and 4.0 are two different numbers.
Now, for anyone who doesn’t understand why the “5 rounds the preceding digit to even” rule is important, I’ve got a simple example that should make it clear.
Using the “5 rounds the preceding digit to even” rule:
1.5 → 2
2.5 → 2
3.5 → 4
4.5 → 4
If you take the average of the original numbers (1.5, 2.5, 3.5, and 4.5), you get an average of 3.0.
If you take the average of the rounded numbers (2, 2, 4, and 4), you get an average of 3.
See? The data has not been skewed using this method.
On the other hand, if you use the “5 always rounds up” rule that most people are used to, you would round in this manner:
1.5 → 2
2.5 → 3
3.5 → 4
4.5 → 5
The average of the original numbers (1.5, 2.5, 3.5, and 4.5) is unchanged. The average is 3.0.
However, the average of the rounded numbers (2, 3, 4, and 5) is different than that of the original data. (The average is 3.5).
See? Following this method, you have introduced an upward bias in the data. This is bad.
That’s why you round to even.
Finally, if there are any non-zero digits after the five, you would always round up. So 2.50001 rounds up (whereas 2.5 rounds down).