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.

I commute 5 miles one way and I no longer "Race my bike" but instead "Ride my bike". On my commute to work I am showered that morning and take my ride slow even while sipping on coffee. Even on hot days I can make it to work with maybe a little sweat on my forehead, but not my entire body. I wear light, hi-wicking clothing on my commute then change into my uniform when I get there.

After work, I change back into my riding clothes and head home. I have to shower after work anyway so I get get as sweaty as I want ;)