In this case, we know it would be incorrect (in general) because eigenvalues are often irrational.
In other cases, there are often better ways to figure out that the answer is correct. Figuring out that the answer is correct using rational arithmetic is often just too inefficient, compared to doing a bit of numerical analysis or figuring out a way to validate the answer you got. That is, unless the problem is very small.
In other cases, there are often better ways to figure out that the answer is correct. Figuring out that the answer is correct using rational arithmetic is often just too inefficient, compared to doing a bit of numerical analysis or figuring out a way to validate the answer you got. That is, unless the problem is very small.