for the guys (and women I suppose) -- keeping the armpits trimmed (via any ole trimmer; I do a #2 length as part of overall general manscaping routine) helps keep the stank down.

To answer the question directly, personally, I prefer a shower after 10km. I sweat easily and muchly. Do I NEED it? Probably not, but it feels better to do it than change into work clothes all sweaty.