It's really about relevance. For example if it is a matter of drawing a graph like [itex]f(k) = k^2 - 25[/itex] and you want to express the fact at the point f(k) = 0 there are two solutions you would probably write [itex]k = \pm 5[/itex].
However, if it is something physical like and related to magnitude you might want to express it as [itex]|k| = 5[/itex]